So I managed to reset my black screen to normal on my 2011 27 inch imac simply by using a labtop in target disk mode to boot up the imac.
1 Put your macbook into target disk mode and connect to imac using firewire or thunderbolt 2
2 boot imac holding down the alt key
3 choose to boot from your laptop drive
4 it should work
5 shut down and disconnect laptop then boot as normal
6 if it doesn't work either try baking or swapping you graphics card or just give up and throw your imac out of the window in a berserker rage