Des Moines, IA: I was having a problem accessing YouTube on Safari. I used to get a black screen. I tried updating Safari Application, updating the flash plugin on my mac. It didn't really work out.
Here is a workaround:
Open the url
http://www.youtube.com/swf_test.html on a new window. Right click on the video screen and select 'settings'. You can see a check box 'Enable Hardware Acceleration' on Display tab of the small pop-up window. Uncheck the option.
The alternate option is to Quit Safari application, Open 'Applications' folder in the Finder. Right-click on Safari, select 'Get Info', check 'Open in 32-bit mode' option.
Again make sure you have latest Safari, Flash Plugin on you machine.