The same recommendation applies to blacklist blacklisting channels instead of individual videos. If a video is determined not appropriate the channel most likely isn't either.
As with Video Rules, the best method to filter sites is to recategorize them rather than adding them to the Custom Allow/Block list. The custom lists have their uses, however, most of the time recategorizing sites will accomplish your filtering needs.
Default Rules - Custom Allow/Block List
There is a 500 entry limit for the Custom Allow/Block List per Group/OU. To stay under the limit use the lists for specific URLs and not entire websites whenever possible. For example, if you want users to be able to access a specific page on Facebook, but not the entire site you should enter in the full URL for that page instead of "*facebook.com*" or "facebook. com*". This also works for certain sections of sites. Continuing with Facebook, let's say that The Virtual ArtroomArt Room's videos need to be allowed, but not all of Facebook. To do this, enter in video selection URL followed surrounded by a wildcardwildcards. This will allow all the videos from The Virtual Artroom's Facebook page.
Please note that will allow the video page, however with video players there could be an asset running in the background that may also need unblocked.
The Custom Block List works in a similar matter, if you want to block an entire site it is best to recategorize it to a category that is already blocked. If there is a specific page or section of a site you want to block you can add those specific URLs to it.
Custom Allow List Examples:
Section of a site: *facebook.com/pg/thevirtualartroom/videos/*
Specific URL: *facebook.com/thevirtualartroom/videos/1371201176544670/*
Please note these examples will only allow the page/section. There could be assets running in the background that may also need unblocked.