OD and defense in general affects the quality of shots taken.
Shot blocking mostly determines whether a shot that's taken will be blocked.
So the two are distinctly difference: against a player with higher defense skills, you'll get worse quality shots; against a player with high shot blocking, your have a higher chance of getting your shots blocked.
It is probably easier to block poor shots rather than good shots, though this is pure speculation.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."