The reason I was curious about theback side of the board, was in case something was shorting it from behind... like an extra unused standoff.
I can't imagine how the power supply could cause that. There is no electronic component in the slot that could have had a voltage surge. The slot is nothing more than a bunch of open contacts waiting for a device to be inserted. With nothing in the slot, there is no load. Although... I can't tell for sure if those are tiny diodes just above the slot or not. On second glance, it almost looks like there may be diodes in that location. If those are indeed diodes, one of them could have failed. But it looks more like the arc occurred inside the slot.
That's not to say that the short circuit may not have damged the PSU if it is an inexpensive unit, though. On a quality unit, it should have short circuit protection for itself.
If you feel the PSU is a low quality unit, it would probably be wise to get a better one any way...