Oracle Text

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.

Card Rulings

7/1/2013 The counter will be put on Witchstalker before the spell cast by an opponent resolves.
7/1/2013 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.