If Cohenim took African wives the Cohen gene (which must be some powerful gene) would be there but the descendants would not be considered Jews... by other Jews.
If what you say is true then you are saying that Jews do not admit converts? For example, if I decided to convert to Judaism I would never be accepted as a real Jew?
What if a Cohen Jew married a black African woman. And then his son married a black woman and then his son married a black woman and this one's son married a black woman. But then suppose that this last one's son married a Jewish woman with pure Jewish lineage... It's a little convoluted but you could get an individual with the Cohen gene with a direct Jewish line on the mother's side that had a lot of non-Jewish ancestors and who physically looked Negroid.