Definitely keep an eye on his stools... It could have been the blueberries, but blood from the upper end of the GI tract can also make stool black. If it's still black tomorrow, give your vet a call.
Steroids are an appetite enhancer, so with luck, when he's weaned off them, the gotta-eat-everything-I-see behavior will taper off!
Might not be a bad time to start "Leave it" and "Give" training, too.