Taken from the practice of leaving a car which has been thoroughly stripped up on concrete blocks. Something is said to be 'on blocks' when one comes back and finds it has been ransacked thoroughly.
I told you not to let anyone in--now my whole room's up on blocks!
UP ON BLOCKS - menstruating. i.e. Out of action, a bit like a car in a garage. e.g. "I don't think I'll be in luck tonight lads,the missus is up on blocks".
to rob someone completely unbeknownst to them, to steal everything from someone, to 'rob them blind'. to go behind someone's back when they least expect it and take as much as you can from them; the opposite of 'strongarming' someone.