After watching several of his maginificant movies I have noticed that quite often or always black characters are not featured that much or a virtualy non existant.

I think this is rather odd since most of his films deal wtih the gangster/drugs theme and that surely characters would inevitably come across black gangstes and dope dealers, especially the setting being in NYC. The only black character I saw from Scorses movies was Stax Edwards (Samuel L Jackson) who I think had a rather minor role in the film.
Also in Taxi Driver Deniro was visciously beating up a black man in a supermarket whilst the shop keeper was just casually watching the scene unfold and not giving a damn.

Did he really have something against them or he just thought that they weren't important enough to pay attention to?