Hi Greg!
Welcome to Greekchat! Questions similar to yours have been asked many times on here. If you run a search for stepshows you should find loads and loads of info. Including a few links to other sites.
I did have a few comments though. I may be wrong when it comes to military drill teams but I think that they were pretty much in full sinchronized force before Blacks started to join the armed forces.
But when it comes to military drill teams I don't usually pay attention to individuals but the group as a whole. They've always fascinated me. Especially silent drill teams that do the quick work w/ the bayonettes. My dad did it in his younger days and has the scars to prove it.
I also wanted to point out that non Black high schools have step teams too. Especially in the South. I went to high school in VA Beach and our school had a team that was about half black and half Filipino. I don't know if they partcipated in any Greek step shows but I wouldn't have been surprised since we were right next door to Hampton and Norfolk State.