I don't think anyone thinks a Catholic priest can turn wine into blood on command.
Neither do Catholics think their priests turn wine into actual blood. After all, they're able to see and taste it as wine afterwards! Instead they're dualists: they believe the Platonic Form of the wine is replaced by that of blood, while the substance remains. And they think this makes testable predictions, because they think they have dualistic non-material souls which can then somehow experience the altered Form of the wine-blood.
Anyway, Catholicism makes lots of other predictions about the ordinary material world, which of course don't come true, and so it's more productive to focus on those. For instance, the efficacy of prayer, miraculous healing, and the power of sacred relics and places.
I really don't think that the vast majority of Catholics bother forming a position regarding transubstantiation. One of the major benefits of joining a religion is letting other people think for you.
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.