On the side of my big toe that is a bit odd. Well I haven the seen one like it before lol.
It is not round but sort of square shaped so looks like it might have been a bump but I haven't bumped it (or at least don't remember bumping it). It doesn't hurt partly I think because I have slight numbness in my feet anyway.
However the thing I wondered about was do bruises lose colour when pressed? This bruise doesn't seem to be on the skin but under it so that the top layer of skin moves but the bruise stays in the same place.
They are bleeds under the skin exactly how you have so eloquently described and usually due to blunt force. Many types of bruises mostly trivial, but some indicating more serious damage eg. bruising around the belly button.
You probably haven't remembered because you have a degree of loss of sensation in your feet.
No big deal.