A picture is worth a thousand words. It's hard to say without seeing it. It could be dust or other particles from the outside. Do you drive with your windows open? It could also be from deodorant. Some deodorant brands do leave some white powdery particles. All in all, black leather shows every little spec of dust or particles. It's just like having a black exterior color.
2013 BMW X5 35i
Alpine White | Cinnamon Brown | M Sport
2013 BMW 550i
Titanium Silver | Black | M Sport