Well just upgraded my planted tank from a 20 gallon long to a 40 gallon breeder. In doing so I couldn't find any worms left in the substrate. I suspect my rainbows went around eating them when they'd stick their heads out to gather food from the water column. I'll wait for this tank to become more established before introducing black worms again and update this thread as to the progress.
If you have fish in your blackworm tank, they will feast 24/7!