The Black population didn't increase. It was always 13% of the population. And the biggest spike happened in the 1990s but dropped.

Relative to the white population it's increased massively. Around 1950 the ratio would have been something like 9 whites to 1 black. Now it's probably 4 or 3 whites to 1 black, especially if you discount the boomers (60+) who are irrelevant for most of these types of stats.