Note: the calculations were done on flags from some years ago and I haven't been able to update them, so you'll see some grey cells and perhaps some flags missing (eg Libya's new flag probably makes it in the top 4 black)
Data from http://www.crwflags.com/fotw/flags/xf-csts.html
You should probably remove Comoros, since its flag has changed. The current Libyan flag also looks like it has almost 50% black. Apart from these, I really enjoyed the quiz! Thanks!
I don't know what flags you're looking at. Maybe the shade of blue is confusing you? http://en.wikipedia.org/wiki/Flag_of_Cambodia & http://en.wikipedia.org/wiki/Flag_of_Costa_Rica