GW Bush got 8% of the black vote in 2000, then 12% in 2004. He also got about 11M more total votes in 2004 compared to 2000. Incumbents almost always win reelection unless challenged in the primary.
Trump got 8% of the black vote in 2016. It's low-hanging fruit to predict him winning in 2020; the fun will be predicting by how much.