iPhone 6s with Safari. I started getting that ad yesterday, usually loading, sometimes at least if not always a video (though the sound was turned off; and maybe it was paused). It is at the top in a spot usually occupied by a longer, narrower -- much more benign ad -- like Heartland flags or something.
Today in addition to the big black always loading square at the top I also started getting a more banner type one at the bottom of my screen where I have rarely if ever had one before, immovable and covering part of CF. It is the kind you hesitate to click the x in the corner (the ad doesn't quite go all the way across the screen) to try to get rid of it because you are never quite sure what is going to happen next. I did once though and it then asked if that was the kind of ad/product I wanted to see advertised in that spot.
Also quite a few random reloads today which I believe disrupt what post you are viewing in a thread, like maybe going back to the first unread one rather than the one you were viewing when it reloaded.
Very annoying, all 3 outlined above. That always loading black square at the top particularly doesn't make any sense.
I have only been using adblock for things like that old Walmart ad that would come up, that is, when it was an absolute necessity. The way things are now, I will probably start using my adblock browser for CF a lot more.