What's going to happen will be two things:
1. The "top" HBCUs will get more selective, especially those that heavily recruit out of state and those "elite" privates. That will lead to less space for those students that are on the margins.
2. Enrollments will grow at some HBCUs while overall, Black student populations will decrease across the board. This is because most HBCUs are unable to house a consistent influx of growing enrollments, especially when many have tons of deferred maintenance items on hold because of funding.