I heard somewhere that when an object was used for mitzvah purposes and becomes dirty because of it, that the dirt is not something to be ashamed of -- it's even something good!
Some examples of this might be an esrog that became dirty after being handled by many people to use for the mitzvah, the wall behind the menorah that got dirty from the fire and smoke, and doing kisui hadam (covering the blood of wild animals and birds that were shechted) by hand specifically.
Where does this idea come from?