For once, the wiring diagram is fairly clear. The black lead runs directly to the CDI coil ("electronic box" in the diagram). It's not a ground, it's a power feed.
You should be able to measure continuity (well, a small resistance) from the black wire to case ground, when the wire is disconnected from the rest of the system. If it measures open, then you have a broken wire / connection or burned out coil. Otherwise, if it measures OK but still isn't producing any voltage, then you've got a short somehwere in the black wire or the coil.