Do you like the idea of testing or quizzing before commenting?
Doesn't sound great honestly
Will this help get rid of useless comments alone, or also reduce the overall amount of relevant comments?
I think no more if you don't force this implementation and instead just have your own four question quiz at the end of your post, but then I guess unless it is opinion based, people could just copy other peoples answers. I just talked myself out of this idea but I already typed it as I am thinking it so it's too late, I'm not deleting it.
Edit: I wish I had an idea to offer, I am thinking on this, but quiz itself doesn't seem right if forced. But I guess if each users has a tab in the back end where they can choose if people have to take the quiz or not maybe it a middle ground. At least this way you can see if users would enjoy blogs with or without those quizzes, as I can see different blogs maybe not even desiring this of their users.