Can I still save my solid block of jam?
From the description "Rock-solid" you guess correctly that you have overcooked your jam mixture. The process of overcooking your jam mixture results in most of the water content evaporating. In candy terms, your jam is probably somewhere between Hard-Ball stage to Caramelized stage. 

For jelly/jam you want the temperature around 220 degree F. Hard ball stage is about 250 degree F. Depending on how far your "jam" is into these stages you may be able to save your "jam" by introducing more water back into it. Try and out a small portion of it into a sauce pan and adding a little bit of water to it. If you are able to turn that mixture into something resembling jam by adding water, that means you haven't cooked it beyond saving and you can do that to the rest of your jam. Note that fruit flavors are volatile so cooking further will result in the blackberry flavors deteriorating.

Otherwise treat your jam mixture as a candy.