It’s the northern state with the second-highest black population. I don’t know why it being solidly D would be surprising. Ohio would be solidly D if it were 21% black, too.
This is another example of the typical fallacy of only thinking about the white voters in a state.
Actually, Trump would have won Ohio if the electorate was 21% black. The electorate is already 14% black.
Also, Mike Castle is an example of a Republican that won statewide in Delaware many times... until he lost the primary to the witch.