Thanks. I think you might be misremembering, as there is an option there "No unique features" but that just removes the features altogether.
Weird thing is I'm 99% sure that I had the signs on at the start of this particular game. I've started new games since and there are no black signs. Can't see an option to switch them on/off anywhere.
EDIT: And now they're back. Only change was that I started a huge map.
EDIT2: Pretty sure that Regenerate Map is the culprit. So far totally new games have the signs, regenerated ones not.