P(guilty|African decent) > P(guilty|Eurasian decent) alright, but that kind of evidence is easily screened out. For any non-trivial amount of forensic evidence E, P(guilty|E, African descent) ought to be approximately the same as P(guilty|E, Eurasian descent). (For example, if E points toward the defendant being guilty, even though you'd assign a higher prior probability of guilt for a black person than for a white person, you'd assign a higher prior probability of E for a black person than for a white person too, so that in the posterior probabilities those cancel out.)