Interesting that you've read this, Ally.
I fed my beloved goldfish frozen bloodworm, and, on one occasion, he became very gravely ill for approx a week to the point where many would have euthanised (although I didn't) and I put this down exclusively to the bloodworm and what must have been a bad batch. What I can't recall is how often I fed him on bloodworm prior to that; however, although he pulled through after I tried treating him with treatment for bacterial infection, I never fed him on bloodworm again, so severe had that reaction been.
I couldn't bring myself to feed the tropicals frozen foods until very recently ie more than 18 months down the line since getting them. I still feel uneasy doing so, but realise that they are needing more variety in their diet now, judging by seeing poo hanging from them more often. I feed them one block per week, and have been doing so since getting the frozen food ~5 weeks ago. So far, so good - touchwood.
What I do is put the frozen cube in a fishnet, pour an entire kettle of boiling water over it, in the hope that this kills off anything, then rinse it further for quite some time under the cold tap, before emptying the net into the tank.
It probably is quite rare that there is a problem but, because it was quite severe in my case (if indeed the bloodrworm was at fault), I'm extra cautious.
The frozen bloodworm is likely to be safer than the live bloodworm, however, as it has been treated with a view to killing off anything, so that might be the preferable option. Also, if you follow my extra cautious regime, that should hopefully reduce the chances of any problems even further. An alternative option, which I've tried too, is Tetra Delica food - they're gel-based.