The latest flash plugin versions 126.96.36.199 and 188.8.131.52 have many problems with Nvidia cards. In a previous article I have shown you How to fix the Blue tint on videos with Flash and Nvidia. Another problem is the leaking overlay. This causes flash to "leak" into other pages or solid black backgrounds. This bug is caused due to incorrect colour key being used by flash plugin. Here is how to fix it.
Open ~/.bash_profile or ~/.xinitrc with a text edit and paste the following line
Log out and back in and the problem should be solved.
If you are using Archlinux you can find in AUR libvdpau-git-flashpatch. Compile it and install it and both the blue tint problem and the leaking overlay will be solved.