Magic the Gathering Protection question
Protection only functions on the battlefield; most rules text only functions on the battlefield. The exceptions are characteristic defining text, i.e. 'this card is black' and text which references another zone, i.e. '{2}, discard 2 cards: return CARDNAME to battlefield from the graveyard.'

In your example your opponent would discard both of their cards, despite their having protection from black, as protection only applies to permanents on the battlefield.