Something's definitely going on there. I guess it's because Trump's a known quantity to black folks. They knew him before he shifted to the Republican camp, and have long standing respect and admiration for the man.
Now he's speaking to something that is perhaps the greatest immediate threat to them - namely, illegal immigration. When he says, "They've got to go", Americans of all races rejoice, but especially blacks, who've been impacted the worst by the invasion.
I think it’s that Trump is speaking to the blacks that have been ignored by both the Republicans and Democrats, working-class blacks who don’t buy into the racist-hatred memes of the Sharptons and the Jacksons.