(1/2) I sent this ask to yatorihell but I'd like to hear your thoughts about it since it's more speculation/headcanon than theory and I have nothing to support it (btw, I don't know if anyone come up with this already ._.') Nobody reconizes Yato as Tsukyomi because of his black hair, while Tsukyomi had white hair just like Amaterasu. The reason Yato's hair is black due the way he died in his past life. (continue)
(2/2) In summary, he made a spell to save his beloved shiki absorving their bad karma (which would made them able to reencarnate - Yukine/Hyori/Sakura??)and his hair turned black, in a way to ‘expurge’ the karma from his body. The spell would be Father’s ideia, of course. Amaterasu, untill now, keep thinking her brother exile himself after the shinkis death, which she is the one to blame. She do think Yato looks like her brother but she has no way to know the truth… What do you think?
We don’t know what Tsukuyomi’s hair color is in Noragami, though. The series takes some creative liberties, and Gods don’t always resemble their “real life” versions (Ebisu and Bishamon being proof of that).
I have seen some people mentioning that Yato and Amaterasu’s eyes look similar. Generally, higher deities are shown with those super light, almost empty stares. But while it’s true that Adachitoka have always put a lot of attention to the eyes, I don’t think that’s enough to assume they’re siblings (and we have yet to see Susanoo and Izanagi!).
On the other hand, Gods never change appearance when they reincarnate. The way Yato died in a previous incarnation (if this isn’t in fact his first one) should not affect how he looks now. Not to mention it’s been hinted at that all Gods die by exploding, so, in the same way.
Lastly, I’m pretty cautious when it comes to spells because they can very easily become an asspull. So far, only Father, as ‘the sorcerer’, has shown the ability to perform spells/black magic/esoteric Buddhism. It’s true that he probably has the ability to conceal Karma corruption (like he did with the medicine he gave Kugaha for Aiha?), but I’m not so sure about the idea of a God carrying that corruption. We have already seen Yukine get really corrupted, and that nearly killed Yato. If Yato somehow had the ability to repress that corruption, he would have done it back then.