Silver iodide plates don’t respond to red light. That alone doesn’t cause the problem but it’s part of it. I can’t remember where I read that tho Without special lighting black people became a dark void. You could photograph them with special lighting and different techniques but that mostly wasn’t done. So I think it’s fair to say it didn’t work well for darker skin tones because in order to get a picture with similar quality you had to work a lot harder. I do agree in the later days it was more a specific caused by racism but from the beginning it was more difficult to photograph darker skin tones and no one really cared because of racism. I wasn’t caused by the racism tho.