Last few days of nightly builds (including 149947) have continued to show a black video rectangle when loading H.264 video from YouTube through the YouTube5 Safari plugin. Audio is loaded and plays properly. This is Safari 6.0.4 on 10.8.3.
r149947 is after the fix for bug 115887 FWIW
Nightly 150044 no longer displays this issue. Perhaps fixed by 149989.
OK, thank you for following up!