When miners are mining and producing block hashes, what is the average value of those hashes?
Since SHA256 is supposed to be uniform distribution the average should tend towards 2^255, right?
Yes. The SHA256 should be uniform, so the first bit is zero with 1/2 probability, both first 2 bits with probability 1/4 and so on. So the share is of difficulty 1 with probability 1/(2^32).