There is very little known about the Blue Child, other than it is a small, blue moon orbiting Azeroth. In-game, it disappeared for a while, due to a graphical glitch around the time of Burning Crusade. Tauren claimed that the Blue Child was sent away to explore the cosmos. It returned in Mists of Pandaria, but it was briefly shown in the Dragonwrath questline in Cataclysm, with the only major lore surrounding it.
Once every 430 years, the White Lady and Blue Child align, which is called the Embrace (because it looks like the "mother" is holding the "child"). The moons were in alignment that way when the Titans (or, their creations) empowered the original Dragon Aspects. The way it's worded in the questline makes it sound as if the alignment is absolutely needed for the new Aspect to be correctly empowered (though, there is a voting part as well). There could be some truth to the matter, since Elune is a goddess with real powers used by the Sisterhood of Elune. Perhaps Elune's powers are amplified with the Blue Child's own during the Embrace, which could have some impact on the powers used to empower a dragon into an Aspect.
Now, who is the Blue Child? The White Lady is Elune, so surely there must be a god or goddess associated with the Blue Child (and by extension, the sun and stars - but that's for another time). There are several beings referred to as "gods" such as Al'ar the Phoenix God, Hakkar the Soulflayer the Blood God (who is most likely a very strong Loa), and the Old Gods. Other cultures have god-like beings, such as the Tuskarr religion with Issliruk, Karkut, Tayutka, and Oacha'noa. However, Oacha'noa has a very interesting line when you encounter her in-game:
"Oacha'noa yells: Little
Considering that the Drakkari were sacrificing their Loa for power, it would seem that the Tuskarr "gods" may be Loa (or Ancients) themselves. This would most likely apply to the other Tuskarr gods (though there is some speculation about Karkut being the Lich King, or Bwonsamdi). So, really, there are very few "true" gods in Warcraft's lore, except...
Mazu is a goddess of the sea, who is really only reported by one NPC: Captain "Soggy" Su-Dao found at Soggy's Gamble in the Dread Wastes of Pandaria. When you enter his shack he is standing next to a bust of a Night Elf lady's head. When you inspect the head he says:
The "dog" Soggy's referring to is a Lion Seal of the same name found in the area, used in a couple quests.
Now, this is only one reference to the so-called "goddess of the sea" right? Surely we can't take one single NPC's account as true, undeniable lore...or can we? There are five other references to the goddess, which are two named areas in the Dread Wastes (Mazu's Overlook and the Shelf of Mazu), as well as an obtainable robe (Mazu's Robe). There are also two quests with her name: Mazu's Breath (a potion made by Soggy) and Mazu's Bounty, which relates to Jiao, a leviathan/kraken. It would be reasonable to assume that many fishermen and sailors in Pandaria believe in Mazu, considering there are areas officially named after her on the continent. They are close to the area Soggy seems to live, so he may have named them in her honor himself, or they may have already been named as such.
It should also be noted that he is convinced that Dog was sent by Mazu. Lion Seals (which are different from sea lions) are extremely aggressive creatures normally. They are also usually found in Northrend (mostly Howling Fjord and Zul'Drak), with one exception (Tideborne Bull) which seems to have been removed from Pandaria in its beta (the Wreck of the Vanguard seems to have once been called "Sapphire's End" where the Lion Seals were found). Considering a friendly Lion Seal found his way to the waters near Pandaria (saving Soggy in this ship), it's fairly convincing that Soggy speaks the truth.
Mazu may have actually created the Lion Seals. Elune created the Nightsabers (according to the pet journal entry) and she also created Wildkin/Moonkin as evidenced by an old quest. Certainly, it's feasible that Mazu could have created Lion Seals and other sea life. She could even have something to do with the Vashj'ir Ancients, much like how Elune has strong ties to the other Ancients in Night Elven culture.
It should be noted that much of Soggy's lore for Mazu is the opposite of Neptulon. In lore, Neptulon is known for drowning sailors, and causing storms on the seas. Neptulon has also destroyed cities that have angered him. However, he has shown a kinder side in earlier history with the Stone of the Tides, giving his powers freely to mortals (and in return, they give themselves to the sea when they are about to die). Neptulon was cooperative in Throne of the Tides, as well, considering mortals were helping him drive back the Naga. Mazu, on the other hand, saved Soggy without asking for anything in return, and allowed him to keep Dog around for as long as he wished. There could be some sort of balance between Mazu and Neptulon - one showing the kinder side of the sea, the other showing its savagery.
The most evidence that Mazu may in fact be the Blue Child is the fact that she looks like a Night Elf, according to her bust that Soggy had fished up years prior.
Now, that Night Elf head model has been used (albeit on a much larger scale) in other areas, such as the Ravencrest Monument in Azshara, and a wooded area in Ashenvale. However, Soggy's description of her sounds rather "elven" as well, particularly the "slender arms" bit. It could be that the original statue pieces in Azshara and Ashenvale were statues of Elune or the Blue Child (considering that Kur'talos Ravencrest wasn't even female, and the monument looks to be older than the War of the Ancients anyway). The developers could have chosen any other female statue head (such as the busts found in Karazhan of human women) or any animal statue head they wanted, but they chose a Night Elf woman as the newly named goddess. That, to me, is a large piece of evidence.
The Blue Child is called a child because it's implied that Elune is her mother, which is mentioned during the Embrace. It's not impossible, but it's not directly mentioned in Night Elven culture. Elune's only named child is Cenarius. However, considering the very name of the smaller moon, and the lore surrounding the Embrace, it would be reasonable to assume that if there was a goddess of the smaller moon, it could be related to Elune somehow.
In the Temple of the Moon in Suramar, there were frescoes and murals of "the gods" - and many Night Elves have quotes that speak of "gods" in a plural form. Their polytheistic religion focuses on Elune most of all, but there could be other celestial gods in their religion, such as the sun, the blue moon, and the stars. Much of Pandarian society was shaped by the Mogu in ancient history, and they had shrines dedicated to the Two Moons and Seven Stars. Likewise, the only two female Mogu are sun and moon themed (with Lu'lin having watery powers). The Mogu and Night Elves seem to share similarities in culture - perhaps due to the Titans, or perhaps due to the events around them.
Elune also has no explicit powers over water and tides (which, as we all know, is due to the moon on Earth). Those powers were much more elemental, being referenced to Neptulon in most cases. It wouldn't be unreasonable to assume that a "goddess of the sea" exists in Night Elven beliefs as well among their pantheon of gods. Mazu would be a perfect (and, in fact, the only) candidate for the goddess associated with the Blue Child.
It should be noted that Mazu is actually a goddess found in Eastern Asian cultures. The stories of Mazu originate in southeastern China. Legend has it, she saved her father (sometimes a brother) from drowning out at sea during a typhoon. Later in her life, she became a goddess upon death (depending on the version of the story). There are some similarities in personality with the two Mazus, such as saving drowning people, being goddesses, and having companions by her side (though, they were demons in the original legend). However, Mazu is revered in southeastern China, whereas the WoW goddess is only mentioned in southwestern Pandaria. Mazu's Robe is also blue/seafoam green, whereas the original goddess was always depicted in red. It is thought she was a real woman who was born ~960, and even has a tomb on the island her body washed up on. In history, her name was Lin Moniang.
It's clear that Blizzard took many references (including, simply, her name) in the small cameos of the sea goddess in WoW. However, what do you think? Is Mazu the Blue Child? Does it fit, or is it too far fetched to believe? Let me know on Twitter!