Nope that is how it looks like for me too when it happens.Nooby - that black screen is only in the video frame -whereas you should get a play icon and then a whirling wheel I get nothing , consistently on any site that opens up a video window . Note that changing the java version and then using monkey browser on the same machine the videos play fine
Every youtube I test just show the black screen.
Sadly I fail to remember exactly when this happen.
Si see no pattern to it. I usually just try to get rid of the block
by restarting the xorg. Sometimes that work and sometimes not .
I have noscript on my and have accepted youtube and
those sites that usually are related to it.
And most of them time it just works but when the error or lock happens
then nothing helps.