Actually African-Amercans have a higher rate of schizophrenia than do Caucasians, about 150% higher rate.  The reasons for this are not clear, as it may be genetic or may be due to environmental stressors.

I doubt if the diagnosis is often in doubt, as the symptoms of schizophrenia are usually pretty clear.  But people who live in cities have a higher rate of schizophrenia than people who live in the country.  And a relatively high percent of African Americans live in cities, that could account for at least some of the difference.  I assume that being a Black in a predominantly White America comes with some added stress, also.

Actually, it seems that schizophrenia is not picky on a race or gender, as most of its symptoms arise from environmental factors, and, not to be rude in any way stephen, but it is very possible to misdiagnose schizophrenia, as it is a rare chronic disorder almost made up of symptoms from other illnesses, like depression for example. The only correlation that may possibly be assumed, and i'm not too sure about the African American ratios', are that African Americans may suffer from greater environmental stresses like poverty and broken homes, which in-turn may cause hereditary issues, if thats what your saying, but i think most data shows a pretty even spread of the illness.