If two or more players play a black suit ("trump suit") card during a hand because they don't have the suit that was lead, is it the highest of black suit cards that wins the trick, or the first one played? (I assume it is the higher of the black suit cards.)
Your assumption is correct.
Also...can a player lead with a black suit card before one has been played during the round, or can it not be used to lead with till one has already been played? (I assume you can lead with one even if one has not been played prior.)
You can lead with any card in your hand, including the trump suit.