Black marks, or scuff marks, on leather boots are the result of friction between the shoes and other objects such as walls, the floor, furniture, doors or your shoes brushing against each other. While these marks are often just slight cosmetic flaws, they can give your boots a very sloppy, careless look. There exist many methods to remove the black marks; the method that you use depends on the severity of the marks.
Brush accumulated dirt and debris firmly off the exterior of your boots with a shoe brush.
Rub a pencil eraser against the black marks. Do not scrub them; you are just trying to very lightly "erase" them. This method is effective on very light black marks.
Moisten a toothbrush very lightly with water. Dab a spot of non-gel toothpaste on it. Gently rub the the black marks with the toothpaste and toothbrush until they disappear.
Wipe off the boots clean with a soft damp cloth. This technique is useful for black marks that are still pretty light, but do not respond to a pencil eraser.
Moisten a cotton ball with non-acetone nail polish remover. Rub it firmly against the black marks. This will remove the heaviest of scuff marks from leather boots.
Apply a quarter-sized amount of leather conditioner onto a soft cloth. Rub it in all over the surface area of the boot, particularly where the black marks were. This step is particularly important if you do step 4, as nail polish remover can dry out leather slightly.
Allow the boots to soak up the leather conditioner for 24 hours. Buff dry with a soft cloth if there is any remaining residue.