Hi.
2 possibilities :
1/ having nothing (=empty background) behind your subject, and choosing the settings adapted to the black background shot, = everything would be black without the flash, but the flash shoot the subject, so the subject is ok. as there is nothing behind, it's still black
2/ having an external strobe, and so you can find an angle to light the subject but not the background (and still have the same setting things, and beeing carefull the light will not bounce somewhere and come back on the background).
for some subject is it not possible to have a black background. it is way easier to have it underwater than out, so train yourself underwater directly.
for most of the situation, 1/200 F/16+ would work pretty well.