See Impermanence, Nostalgia, and Parity.
I also considered "Gain control of all enchanted and/or equipped creatures" with the name "Jealously."

Hm. Impermanence is a blue concept with a non-blue effect if you cast it for all non-blue colours. Parity is a white concept with a non-white effect (white being the worst at card-draw) if you cast it for all non-white colours. But on this one, I'd say black is third-best at gaining control of opponents' stuff (and second-best at permanently gaining control of opponents' stuff), so the pattern doesn't work so well.

Mind you, it'll be an incredibly difficult pattern to preserve through all five cards!

You're right. I didn't succeed at this one, yet.

Hmm... so if you pay RGWU, you end up with a 2/2 creature that intimidates for black? The rest of the cycle works, because the off-colors could give you flying, or haste or flash. But I'd suggest swapping out intimidate on this card for... maybe first strike? In fact, maybe the keyword for this cycle should be what these colors are normally secondary or tertiary at?

It has intimidate in a nod to flavor. As in, it's scaring all of the other creatures onto your side.

Looking back, this works much better as a green card than black, since green doesn't steal creatures. But I still can't think of a black effect. What does black not do mechanically, other than destroy artifacts and enchantments?

Looking at the others, the abilities don't have to be primarily associated with what one colour can't do, so much as an effect that's only permitted in two or three colour combinations not including the chosen colour.

So we're looking for, say, a or ability that can be construed as having a flavour.

So perhaps blink? I don't know how you make a rare sorcery version of blink, though...

Or or ... how about "cast a card without paying its mana cost"?