I picked up this mark on my right shin. It's not really a stain, because it didn't soak into the material. I don't know what it was, because it was dry already when I saw it the first time, but it looked like a mix of chocolate and mud.
I managed to get most of it off with my fingers and nails, as you can see from the top photograph. The rest I managed to brush away or off the denim.
The reason I use this brush, is because it is soft, and doesn't damage the denim. It also manages to penetrate the material just enough to get the dirt out, with a couple of hard brushes, leaving the denim as good as new.