I have had them in a gravel substrate and they have done fine. This is my invert tank so they really have not had too many predators. I have also noticed that sometimes they tend to stay hidden in the substrate, while other times I find them all over the driftwood even. They always are in mass supply in my HOB filter (thats really the only reason i keep it since it provides an area filled with muck and they seem to love it)
We are talking about California Blackworms arn't we? B/C I have never seen that behaviour from them at all. Mine just bury themselves in the sand and stay there.
Best wishes,
Wes