The answer given by the small LFS about the worms is pretty marginal at best. So that tells me the information they give out could be limited as well, so be wary.
There are a lot of things that go into this. For a start, I would be interested in the type of fish you bought from each store (having damsels survive from one store, and tangs die from another...is a consideration). I would be interested in specifics of your tank in terms of size and age and parameters. The parameters of the stores tank...acclimation...etc. How the fish died, whether several at a time or individually.
Basically there is too much unknown, but I am quite certain it is not the blackworms.
The size of the store and number of tanks is not, IMO, a good factor necessarily in choosing a store. I've seen some small shops that look horrendous, and big one's too. And often it changes over the course of the year...things happen. If our tanks go through problem periods you can imagine what can go wrong at an LFS.
So I guess the question is...are you interested in "investigating" more what may have happened or was it a question on the worms? If the worms, it is not what is killing the fish (and it doesn't seem it is clear that is what they are even feeding them).