Witchstalker

• WOTC Official Reminder Textofficial rules
  Hexproof — This permanent can't be the target of spells or abilities your opponents control.

• WWizards of the Coast2013-07-01
  Official Ruling
  The counter will be put on Witchstalker before the spell cast by an opponent resolves.
• WWizards of the Coast2013-07-01
  Official Ruling
  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.

Rulings via Scryfall