Blanching is where you shock the food. You put it in boiling water and remove after a minute and put it in ice.
I don't believe feeding minnows is a complete diet and also has a lot of risks with it. You don't what diseases the feeders have, what nutrition they are bringing in, and all of that. I would recommend finding a more reliable source than a chain store and maybe consider a staple pellet if he will take it. Maybe mix in other things too, like frozen foods, veggies, snails, shrimp.