Wow i had a jd that was just like that. Wouldn't eat dry food no matter what. He eventually succumbed to his own appetite and choked on a syno cat. Wasnt quite the stunner your jack is if its the one im thinking of.
I think the bloodworms may have an intrinsic effect on the fishes metabolism and hormones and therefore its well being and growth. While i do agree that a pellet contains all of the nutrients a fish may need to live, there should be at least some food in its diet that bears a semblance to something it might taste or eat in the wild. To sum up, I think the bloody nature of the food simply excites the fish. You can see how pretty much all fish will perk up when bloodworms hit the water.
But i have nothing scientific or proven to back all my theories out. Im no fish nutritionist, i just try to keep my fish happy and healthy.