Hi Stu,
I wonder this about the frozen versions of foods myself. I even wonder it about the freeze dried stuff too. But then I think that maybe its a matter of building up your fishes immune system to these types of foods, I know we keep them in captivity but in the wild they would be eating all types of potentially disease ridden bugs, fish or plant matters. So is it really that dangerous that we feed them these foods, I don't know for sure. Yes I have heard that frozen blood worms can carry diseases, and that you should really only feed them as a treat, not a staple diet.
I recently lost a discus that developed what I think was hole in the head, white pimples all over his face. I'm still trying to work out how he got the disease in the first place, for the first month I had him I only fed him tropical foods like flakes and pellets, then 2 weeks ago I order my first bag of freeze dried black worms cubes and started feeding those daily, within 2 weeks he had the white pimples on his face, I guess its made me wonder.