Re: Ground wire confusion?
I can't believe no one has answered your question directly.
Both are ground. You have a "chassis" ground, (the bare wire), and an electrical ground. (The green wire.) However, you may want to connect the pickup out-of-phase. Then the black wire might be "ground". Actually, "grounded" would be technically more correct. So . . . the bare wire, (chassis), is
always grounded, while you can select which of the other wires becomes "ground", based on you wiring scheme.
Make sense?
Edit: Btw, my answer is only more clear than the others, if you use
my logic. Not everyone does.
