Witchstalker

Hexproof (This creature can't be the target of spells or abilities your opponents control.)
Whenever an opponent casts a blue or black spell during your turn, put a +1/+1 counter on Witchstalker.

The List

Magic 2014

If an opponent casts a copy of a card that’s blue or black during your turn (as opposed to putting a copy of such a spell directly on the stack), the ability will also trigger. Check for the word “cast” in the effect that creates the copy.

The counter will be put on Witchstalker before the spell cast by an opponent resolves.