I don't know how informative this will be but on this topic
http://www.google.com/support/forum/p/Chrome/thread?tid=2929fa04fadff0c3&hl=en that was closed, there is another answer in regard to playback. I was getting the black box and there was no video playback. I tried every tip on that thread and none worked. Then I tried the suggestion by videosooner to add
https:// before the youtube url. That worked but I had to do this manually each time. So I cleared the cache as the others suggested and this time... it worked. The answer was to add
https:// and THEN delete the cache which now seems to have cleared the problem up.
Essentially, Google itself or Youtube made some security changes apparently and you have to teach your browser about the change by manually adding the https:// videosooner suggests and then clear your cache. This just happened to me either last night or this morning but the fix seems to work.