Runningback holes are specific areas between the linemen in which a runningback is supposed to run through. The holes have specific numbers and are told when calling a play. By doing so, the runningback and linemen are prepared to assist the runningback through.
There is no such thing as running "black" holes, so I believe that you mean running back holes. Running back holes are gaps in between the linemen that the running back is supposed to sprint through. The running back is an offensive position.
Linemen in football are designated to move the opposing team's linemen in order for a running back to break loose carrying the football. The space that they open up for the running back is called the running back hole.
The running back holes are the area that is clear of defensive players by the offensive players. On each play the offensive players are given a play where there is a designed spot for the running back to break through at.