The population of the US is made up of 13% of blacks. Lets assume that blacks make up the same % of voters. Historically, republicans will get 10% of the black vote. If Trump doubles that (from 10 to 20 percent), thats 10% of 13% equaling 1.3%. Thats 1.3% more votes that Trump will get but also 1.3% that hillary won’t get. That is a 2.6% shift!
And we are only talking about one demographic here. Hispanics, independents, Reagan democrats, etc the same thing can happen.
Except the Republican figure is not 10% but more like 5%.
And, the other black number is those that voted for Obama but will not vote for Hillary because it’s just too much effort.
The two factors together will be a much larger % than the 1.3% you calculated
Republicans have been averaging closer to 5% in recent elections of black vote. Reagan got a bit more.