Eh. One source of food is never healthy for any animal; variety is important to fufilling all of an animal's dietary needs. The pellets have the minimums a betta needs for survival, but blood worms, fruit flies, brine shrimp, and other live foods offer nutritional qualities that can't be packaged so easily.
If you think about it, no carnivore consistently eats the same thing day in and day out. I'd think that for them, varied protein sources would be very important. I've never heard of feeding an all-bloodworms diet, and maybe it works, but it just doesn't ring of safety in my mind based on the principles of animal nutrition.
If they're that cheap, I'm guessing they're freeze dried blood worms? If so, that can be dangerous; it can cause swimbladder problems, and constipation. I suppose soaking first might help.
The Petco near my feeds only freeze dried bloodworms; half of their bettas have swim bladder problems, they're floating all over the cup. And none of them look healthy.