There is a valid argument to be had that, in the United States, only whites are "racist," but that argument depends highly on how "racism" is defined. Using the simple dictionary definition, that racism is making negative assumptions about people based solely on the color of their skin, yes, of course blacks (and latinos and other minorities) can be racist. But in conversations on racism, there's frequently a more powerful definition at play: Racism is (in that view) using social power and traditional attitudes to make sure that certain privileges stay with people of a particular skin color. From that perspective, it's much less clear that blacks can be racist (although, to rebut myself, it's not impossible, depending on what "certain privileges" we're talking about -- certain members of the various "oppressed" classes have learned over the last decades how to leverage SWAMP* Guilt to get special treatment).

* Straight White Anglo Male Protestant, my own acronym.