Doesn't the necromancer robes have a hood? I can't really remember ever seeing one, but if there is a necromancer hood to match the robes it would be black.
You are right, there is Hooded Necromancer Robes, even Hooded Black Mage Robes (Synod). The problem is that they enchanted already, so you can't put your own to them.
I'm sure this has been asked here before, but are there any black mage hoods in the game that are obtainable on a console?
No. It would have taken maximum 2 minutes of hard work for Bethesda to make it, but here we go again ...
By the way, just in case you didn't notice yet, every Expert and Master Robes' modell are the same as College Robes. Buy those and an (unenchanted) Mage Hood at Radiant Rainment (Solitude), and you'll be fine as far as outfit. Note that the color of the robe's "belt" and collar are the same, or at least identical to the hood's color. This is important because the aforementioned robes' mesh conflicts with _any_ hood - in that case too, but because the similar color, you won't notice it ...
I guess it was a designer decision, that's why we never had Expert or Master Hoods from default.
So from my personal experience, this is the only way wearing College / Master / Expert Robes and a Hood. I can't figure out a same solution for black robes, since we haven't got any separated, enchanted black mage hood in the game ... Its like gloves, where at the end of the day, we have one type for regular use. Its a shame, really.