I just came across this thread while looking at how to run a couple fans from a 2011 iMac 27 inch that we found in the recycling.
On the BFB0812HD -HM01 (610-0035) blower (optical drive/GPU): With label side up, in order from the 'bottom' of the label up:
VM, OP, GND, VCC
On the BFB1012MD...