Black preachers deal with the day to day consequences of insobriety; the bruises, cut lips, broken bones, bullet wounds and the wasted lives. So, no, they're not going to be easily persuaded that ending any form of prohibition benefits their parishioners and communities. Especially when every corner store offers 20 varieties of totally legal, high alcohol content beer but not one piece of fresh fruit. Prop 19 isn't going to help that ratio one bit.
If you are going to win over church going people you're going to have to explain the consequences of prohibition and how that does more harm than good. It's a tough sale, though. Be prepared to lose 8 out of 10 God-fearing people who see prohibition preventing a bad situation from being even worse.