Youtube videos appear as black box in my blog posts. They also appear as black box in the WordPress help section under Support Topic "Media." I'm having this problem using my Windows 7 machine. In the past this problem did not exist. I do not have the problem using my Windows XP machine.
I can click in the bottom right corner of the black box and the video will occupy the full screen, and I can then play it. But when I press the escape button the full screen reduces itself into the little black box on my blog post again.
Blog url: http://jlippinbike.wordpress.com/