I've never seen these robes in Skyrim before.

I got a "Robes of Minor Destruction" out of a chest, except they're black Greybeard robes with the necromancer emblem on them. They look bloody brilliant.
What is the base item called and where can I get it?

Basic Black Hooded Robes? Show up as loot and if it is base clothing, Radiant Ramient (sp?) in Solitude has almost every article of clothing in the game.

I've found some that just say "Necromancer Robes" that are hooded, but they come pre-enchanted.

I've seen them, but it was a long time ago on a different character.

I found one on the boss mage in Morvunskar. PC, but was using no mods back then.
I think they were just called "Black Robes" and had a 50% magicka regen. on them.