As reported by @ruimams in #298:

Well, in case Ken-g6 wasnt clear, this is not very effective:

That's a video stream with a blocked ad over it.
Even if I set "Blocked frames color" to transparent, the box that says the domain of the blocked frame still appears.

EDIT: Ok, (I think) I now see what you meant. Anyways this is a feature request so it's your call. In my opinion it's a good idea to have that option (it could be off by default).

URL where the problem is highlighted:

Sure, www.ptcanal.com/tvi.php.