UPDATE 2 It seems that starting today, uBlock Origin is working to combat this Youtube Block. Mine started working again! Lets all thank the devs of UBO for fighting this fight!

UPDATE So as new info comes out, I'll be posting it here. It seems as if this Rollout Has Several Parts.

Part 1

You get a popup message over top of your video, blocking the screen:

• This is the first sign. If you see this popup AND are logged into a YouTube account, your account has been selected.
• At this stage you can likely close or block these messages with an adblocker.

Part 2

This message will change, indicating that you have 3 remaining videos to watch without ads.

Will insert photo once one has been found

• At this stage your adblocker will imminently stop working in 3 videos time.
• Personally using Firefox + uBlock Origin and tweaking filters and updates does not even fix it.

Part 3

None of the video loads now, everything looks blank.

• At this stage you must tred new ground to avoid ads. I have posted methods in the comments. If you want to bypass this end page, read down there.

End of Update

YouTube has started rolling out anti-adblock to users inside the United States, which means that they are preparing to roll this out to the entire country. Personally, I have been blocked already. I want to gauge how common this occurrence is.