Ask some random magic player what colour "exile a creature from a graveyard and make a zombie token of it" is, and 19 out of 20 will say black.

Hour of Eternity makes complete sense at XXBBB..... why on earth would this spell be XXUUU?

Choose something blue for the blue hour!

And the mummy making on this world is white and black. You're not helping your case there. And this is still true:

Is it bad that I prefer to think of Hour of Eternity as blue/black, rather than blue OR black? It's much the same way that I think of, say, Honored Hydra as green/white. My own particular take on color identity classifies token color as part of it.

And the mummy making on this world is white and black. You're not helping your case there. And this is still true:

Yeah because you're qualifying it with ZOMBIE rather than TOKEN. The Zombie portion is inherent to Amonkhet and is entirely flavor based. You're intentionally asking a leading question rather than asking based on the ability. As others have said, popular opinion doesn't change that the ability is fully within blue just flavored as a zombie card.

I'm with Epsilon on this one - Back from the Brink is blue, and mechanically this is very similar.

I don't think that's the best example. Back from the Brink doesn't match blue's colour pie. It was allowed because of the flavour of the Innistrad set.

When the Council of Colors looks at cards to see if they are acceptable within the color pie, they give each card a grade it from 1 to 4. A grade of 1 means a card is within the color's normal slice of the color pie. A 2 means it's a bit of a stretch but it makes sense in the set that it's in. A 3 means it's a bit of a stretch and doesn't fit within the set it's in. Finally, 4 means it's clearly not in that color's slice of the color pie. For shorthand, 1 is "normal," 2 is "an acceptable bend," 3 is "an unacceptable bend," and 4 is "a break."

Hour of Eternity is a great example of the separation between a 2 and a 3, or an acceptable and unacceptable bend. Blue traditionally does not mess with creatures in graveyards. Blue usually gets back instants and/or sorceries and artifacts on occasion. When we made embalm, our solution to solve memory and counter issues was to have the card make a creature token that's a copy of the creature card. Cloning is pretty squarely in blue, so it pushed embalm more toward blue.

In Hour of Devastation, we tweaked embalm with eternalize. Also, we tied the Eternal army to Bolas's three colors (blue, black, and red). Hour of Eternity essentially eternalizes all the creature cards in your graveyard. Eternalize is partly blue. Some of the Eternals are blue. Cloning the dead is kind of blue. In this place at this time, the mechanic made sense in blue, but if we tried to make this same spell in another set, it very well could shift from a 2 to a 3 and not be printable.

So yes, this is blue messing in black space, but in a way that, for this set, makes sense.