This should be blue, since it's really just Time Warp in two player. Stranglehold makes me think it could almost be red.

It is Time Warp in two player, but I'm not convinced that means it has to be blue. I can definitely see black, and possibly other colours. Maybe in white: it's sort of a tax, and white has the Silence effects...

Does blue get anything uniquely its own any more, then?

I don't think there's anything that absolutely never appears in more than one colour, but counterspells are probably the closest (having appeared in other colours in extremely specific situations, but normally confined to double-mana) and appear in blue. After all, other signature abilities like mana ramp and direct damage which don't normally appear in other colours do appear on expensive enough artifacts, but there's not really artifact at any cost that does what cancel does, unless you can cheat cancel onto the stack.

I think abilities which blue is practically the only colour to have are: extra turns, counterspells, milling, bounce, flood, direct card draw, and permanent mind control, which is pretty good. Extra cards, deck manipulation and counterspells are why so many people find blue powerful even though it doesn't get the best creatures.

Targeted discard is probably close to countermagic in terms of unique-colour-ness.

But sorry, I may have been unclear. Time Warp effects are pure-blue. (Red gets extra turns but only in the Final Fortune mould.) But something that works out very similar to Time Warp could still be nonblue, in the same way that "Creatures are unblockable" is blue while "Creatures can't block" is red, despite working out exactly the same.

That was my comment.

Oh yes, sorry, I should have said, I agreed with your assessment, that "another turn" effects are blue-only, but functionally equivalent ones like "target player skips a turn" make sense in black.

"Targeted discard is probably close to countermagic in terms of unique-colour-ness."

Good point. I would have guessed that there were a few non-black effects that targeted a particular card sort or something, but it seems there isn't.

(But this is closer to the famous original wording of Time Walk...)

Anyhow; yeah, with enough flavour anything is pushable to any colour. you, uh, might not want to cost this as cheaply as Time Vault did, though :)

I think it needs to fit in terms of colour identity (eg. white gets lifelink and green gets lifegain), colour philosophy (eg. white doesn't get unconditional removal) and balance (eg. red and black don't get enchantment removal for less than about 7).

In a way, black already gets this effect, only better: See Worst Fears and Sorin Markov.