Black Hole Factories are buildings that only appear in campaigns, and unlike other buildings, they cannot be captured by infantry. They make their only debut in Advance Wars 2: Black Hole Rising. They can build any unit, sometimes more than one at a time. They don't use up any war funds and are powered by Black Hole Pipelines. However, destroying the Pipe seam will shut down the factory and end the mission in victory.
Blocking the entrance of the factories prevents them from producing units. This technique is also used on bases, ports, and airports. This leaves your other units free to attack the seam and win the mission.