Some of them are -- but I have, on many occasions, sat in what you would call "black churches" and listened to sound Christian doctrine expounded.
Here's a suggestion, if I may. Visit the churches in your area which have predominately black congregations. If the "preaching" is too political move to another until you find one which preaches Christ and the Gospel.
Maybe even join. You won't regret it.
If that were true, then they would not so completely and utterly, reject Christianity when voting.