Have you looked closely at a picture of Anwar Sadat?
I will never forget in my first year in HS (which I went to on an el train) seeing every third person with a smudge on his/her forehead one (in retrospect) Wednesday morning. I thought I was losing my mind.
There’s a fourth behind the tall guy in the front, to the viewer’s right of the guy in the red shirt. His head is bowed more, so it’s harder to see, but it’s there.
Could it be, that in the act of praying in their specific location (out on a ‘dirty’ public area) with what might be increased enthusiasm on their parts due to circumstances, that they act of placing their foreheads on the ground simply dirtied them instead of it being a permanent or semi-permanent mark?
It really does not look like dirt to me.
You know, now I’m thinking maybe they did put their heads down on something that stained them accidentally. There are more pictures today of men praying on the ground and it looks like they are grabbing whatever is at hand to put on the ground where their heads will touch – a piece of paper, anything. They’re in Tahrir Square and there’s dirt and chaos everywhere, and they don’t have their regular supplies and are trying to make do with whatever they can find. Maybe they were using some kind of wet cloth or something stained with ink, and the dye came off on their foreheads.