I've noticed in the last few weeks that YouTube has some sort of fallback when an ad gets blocked. You get a white overlay with with impossible-to-read lightgrey text on it, with a blue button in the middle with a call to action on it (eg Signup Now, Click to Learn More). There is still a Skip Ad button and a widget to indicate when the ad will go away on its own.

I have two YouTube accounts, and at first it only happened on one. Now it is happening to both. Maybe it is something they are currently just testing.