Black opening for 500+ elo

I recommend: if 1.e4 by white than e5 (Kings pawn opening) or c6 (Caro-Kann), if 1.d4 than d5. But deep opening theory at that level is not that important. You just have to know the opening principles.

Defend 1 e4 e5 and 1 d4 d5 as black and open 1 e4 as white.

The best opening is the one you are most comfortable and familiar with. It's important to study and understand the ideas and plans behind the openings you choose to play.

Yep already been suggested as black v Queens pawn setups use a slav setup 1...Nf6 followed by 2...d5 and 3...c6 and has black v 1.e4 use the Caro Kann 1...c6, followed by 2... d5 Book recommend is Caro Kann the easy way by Thomas Engqvist