Is my black screen the cause of my video card or power supply?

Video card. No question.

If the power supply is rated sufficient for the model of video card you have and was connected correctly then I'd guess your video card overheated, and if it wont' work at all now it may be permanently fried.

Most video cards I've bought lately have fans that auto adjust their speed, supposedly, to compensate for additional heat when the card's put under a load, but I've generally found the stock fan control settings don't run the fans up high enough to keep the card cool, and they'll often lock up under load unless I override the fan control and run the fan speed up to something reasonable, like 70% or more, while I'm playing some graphics intensive game or other.

There's a few different utilities that'll override fan controls like this for a lot of different cards. I've been EVGA's Precision program for that, it's real nice.

if the wattage decrease than what it would be, it could desire to no longer artwork even though it is not gonna instruct a green line, the golf green traces may well be a broken video card from over heating. appropriate to the 450watt potential furnish i think of it is better than adequate for 8800GT yet to confirm you're able to desire to examine the video card handbook. the low wattage won't reason a failer in video playing cards.

Power supply