There's this extension called Intention which, I think, handles these edges cases pretty well.

It doesn't really block websites, it just pops up in the middle of the screen warning you about how much time you've spent on that particular distracting page today. But you can mark a checkbox telling it that you will be using the site productively (watching a lecture at YouTube, for instance) and assign a period of time for it.

I've tried a lot of websites blockers that didn't stick but I'm really digging the approach of this one.