"Rub the bottom of your pot on the ground for 5-10 seconds. Sand, grass, whatever. That gets rid of the residue."
Maybe. Maybe not. I've used three different brands and packages of Esbit fuel this summer, and I found each one to be different. With one, the black residue would rub off easily. With another, it was more persistent. With the third, it was tough. Part of that has to do with the height of the cook pot above the Esbit, the metal texture of the cook pot, and the air temperature. If you rub it off quickly, it will come off easier. If you let it harden, it is tougher. I got to where I would not even try to clean it off in the field. I would just throw it into a produce bag (to keep the black from rubbing off onto everything else) and clean it at home.