Locking the black hole controller does not block the input of empty black hole units. For example:

Shift clicking a stack of stone will insert it into the the first empty unit, rather than the unit locked to stone.

Also, slightly unrelated, completely emptying a black hole unit inside a controller will still keep the nbt tag remembering the item. Meaning if you want to change what item was kept in the unit you have to empty the items out, take the unit out of the controller, and place in world and break to clear the nbt.

Industrial Foregoing Version: industrialforegoing-1.12.2-1.8.10-143
TeslaCoreLib Version: tesla-core-lib-1.12-1.0.12
Crashlog (If Any): n/a