We have received many inquiries about videos hitting a black screen and not playing. The culprit has been discovered and I'm happy to share our findings with you!
AdBlock (and AdBlock Plus) released an update that now defines the scrolling "marquee" text at the top of some videos as an ad and they are blocked by default. This stops our player from continuing video playback.
You can fix this problem and continue to watch videos on Niconico by doing ONE of the following:
1) Disable AdBlock completely
2) Go to the Niconico homepage, http://www.nicovideo.jp/, click the AdBlock (or AdBlock Plus) icon in your browser, and click "Don't run on pages on this domain."
3) Use the English Version of Niconico! The scrolling text only appears on the Japanese side, so feel free to use the ad free English version if you don't want to change your AdBlock software.