You have to be careful on internet and this is laced with sarcasm of course. If you mention that a person is Black or Black people in general, then somehow you are by default a card carrying member of the Ku Klux Klan. If you want to avoid that you can start sentences like this - "Like me, they are _______ Americans". Some people haven't learned yet that you can describe a person by race, not actually being a racist.
Remember everyone... Black is an adjective. A descriptive word. It is not racist until you make it racist.
Lastly, I understood what you meant. African Americans are more likely to catch the virus and they should be aware of that therefore avoiding large gatherings such as the one you witnessed. Not only were you NOT being racist, you were concerned for them and others they may be in contact with. By the way, I saw this very same thing while delivering pizza. I went right past the same kind of party, BIG SLIDE, lots of people however the people I saw were inbred rednecks. There were 4 or 5 really well built chicks in bikini's though.