And I also found this,Adobe Flash Player 10.2 has a issue with YouTube:
beermug - Feb 21, 2011 2:34pm GMT
THIS seems to be an incompatibility with youtubes main screen video viewer and Adobe Flash player ver 10.2
I have windows pc and same thing is happening. The new Adobe ver 10.2 isusing something called StageVideo API . It is written so the demand is much less on the CPU and does the same job.
Ive been scouring google for days now, and i've seen some temporary solutions that work. for windows pc's you can click on the rectangular button on the right hand lower corner in the Youtube main video player and the video popsout on the screen in a seperate window and plays normally.
I understand that MAC users cant do that ,because those control buttons dont seem to be visable. However, you can click on the name of the person that uploaded the file and go to their Youtube channel page and the video will play in the channel video viewer. This also works on windows pc's.
Some people are saying that they can view videos with one browser and not another, i have IE8,Firefox ,and Google Chrome ,and i get the black screen on all of them.
Youtube seems to be temporarily advising to disable the "hardware acceleration" feature in Adobe Flash player. If you do that, then you can view videos normally. The downside is that you are making the CPU work alot harder, thus defeating the advances of the ver 10.2 flash player. Personally id rather just wait until they fix the issue.
To disable the hardware acceleration you can get the instructions here:
So those are some choices that i have found. I sure hope that Youtube can fix this problem soon.