This morning we resolved a problem that prevented Flash Player installations from completing for some of our Macintosh users.
If you encountered this problem, please delete any previously downloaded Flash Player installer and either:
1.) Download the installer again from https://get.adobe.com/flashplayer
2.) Download the stand-alone installer from http://fpdownload.macromedia.com/pub/flashplayer/latest/help/install_flash_player_osx.dmg
still does not work
I thought when I read your note that you were saying that the installer just shows a black box, but on second read, it sounds like you're downloading and running the installer successfully, but then you go to surf the web and you just get a black rectangle where the Flash content should be.
Youtube is mostly HTML5 (unless you're right clicking/ctrl-clicking and seeing About Flash Player), which might explain why it's working.
Are you using any ad-blocking or anti-tracking plug-ins? They can often times prevent Flash content from loading.
Some more details about your computer and browser would be super helpful in giving you better advice: