Well first, what's a 'regular sand tank'? They need at least 2-3" of sand in a well planted tank and a good food source (mulm) that most display tanks tend not to have. That said, everything I've read is that the worms become a sought after food source that don't stand the test of time. Also, I think blackworms are sold in quantities that tend to be much greater than most would realistically put in a single aquarium. You might need to establish a secondary environment for them to live and perhaps breed. Live foods are somewhat cool, but come with housing and maintenance requirements.@AbbeysDad
Would it be beneficial in a regular sand tank to introduce populations of blackworms, as a continuous breeding food source for the fish? I'm intrigued.