Corrine McConnaughy has researched it:
Ismail White and I have been conducting a number of studies on the uniqueness of attitudes toward black men. In a nationally representative sample of white Americans, we find that black men are indeed considered uniquely violent. While a traditional question about racial and gender stereotyping finds that whites perceive “blacks” as more violent than “whites” and “men” as more violent than “women,” a question that asks about combinations of these identities—black men, white women, etc.—shows how black men are uniquely stigmatized. More than 40 percent said that many or almost all black men were violent, but less than 20 percent said that of black women and white men. The figure [above] displays these results (with the bands indicating 95 percent confidence intervals).