Experienced this "black" screen problem on my late 2009 iMac. It started a couple of months before it was booked in to have an SSD drive fitted. It happen twice a day initially, but by the time it was taken away to have the new hard drive fitted it was every ten minutes or so. During the SSD install it also had its logic board replaced due to faulty connector and upon return the machine is so much faster, but the black screen problem is still there. Done the PRAM resets and the other one and turned down the screen brightness ( which helps prolong the cut out time but still not a permanent solution). Tried fan speed software, but no better.
Decided this morning to look at energy saver settings and discovered they were set at 5 mins for display and computer sleep. I increased them to 1 hour, ignoring the warning about being more costly to run and also wearing out the display and drive sooner as I wanted the thing to work!!! and voila the black screen has vanished. I've even turned the brightness back up to full which previously seems to crash the screen to black and no problem.
So don't spend money on software, or hardware try this simple adjustment of your "sleep" settings in System Preferences - Energy Saver first and see if it that resolves it for you. I assume an IOS software update must have altered these settings at some point as Ive never been into Energy Saver before
