There is a flavor reason for that, but it's secondary. The primary reason is mechanical.
So, mechanically: deathtouch is firmly a part of green and black color-pie-wise. Since deathtouch is so hyper-specific, a colorless Gaunis would've needed a cost bump to 5 cmc, making him much weaker than a 4 cmc with the same effect. Thus, the color heaviness in black allows it to hit the board a turn earlier.
Flavorfully, the point is this one. Yes, the Eternities do have an influence, but said influence is limited to two of his abilities: the passive (impervious to noncombat damage) and the ultimate (plane-consuming annihilation). The +1 ability, as his origin flavor text indicates, is entirely innate and actually trainable for ordinary people. And it's what gives him his blackness.
Now, I'ma go design some cards to showcase his arsenal/powers.
EDIT: Done.
