I've the same problem on an iMac mid 2010 i3 3,06 Ghz : power on, i hear the bong and the sound of the fans and apparently nothing more.
But, i tried to replace the hard drive with a SSD one that i installed on a macbook pro, with an important precision : in this fresh install the session is automatically open.
When i start the iMac, the boot sequence is still with black screen but once the session is up, the screen appears. Very strange.
I tried to switch the screen with a spare : same result.
I'm blocked at this step. The problem is half solved, i would know what is the problem exactly, i don't understand from where goes that.
hope it helps you