what are good openings for black (-600) elo?

I like the French Defense & Queen's Gambit Declined.

I think it's more important to be knowledgeable about opening traps, because people will definitely try them at your level.

READ: Learn them to defend against and avoid them — NOT to play them.

For example, here's a common trap in the Scotch Game + how to refute it:

Here's the Elephant Trap (not to be confused with the Elephant Gambit) as well:

There are plenty more, of course. I'd recommend looking up the Legal Trap as well, as that one can occur fairly often as well (not in my experience, but still).

At your level openings are not deciding your games.

Just follow opening principles.

Blunder check your moves.

Take your time.

egg gambit