As for the batch of "all black males", try putting individuals in a jar and using a flashlight to "candle" them. I wonder if the gravid spot would show a little differently from the tail coloring on females.

Getting a whole batch of one gender has got to be quite unusual (unless maybe temperatures or hardness or pH were really at an extreme high or low). If they are all male you are either very lucky or extremely unlucky. Enter a lottery, but stay out of thunderstorms. ;)

Thank you, by the way, for the photos and video referred to in your last diary.