Bava Kamma 20: Blackened Walls Means Rent is Paid

Amud (a) of today's daf provides us with a number of examples of animals causing damage to other peoples' properties.  The examples sound similar to cases that might be brought to court today: a goat climbs on a barrel to get a turnip; the owner of the goat is liable for half of the cost of the barrel in addition to the turnip.  And so we also learn about Eating - climbing a barrel is not typical behaviour except when it comes to a goat's appetite.  And damages that involve Eating incur liability of half of the damages incurred.  

Other examples include cases that involve Eating and Goring and subcategories of each.  Amud (a) ends with a discussion about squatters.  What is done when a person lives in another person's courtyard without paying rent?  The Gemara wonders: does the owner actually suffer a loss if the property wouldn't have been rented out otherwise?  And does it matter that the squatter derives benefit from the owner's property?  And what if the squatter is an animal?  And what if the courtyard is deemed ownerless?

Amud (b) reminds me of Masechet Eruvin, where we discuss the boundaries of a home and measures of the inner and outer portions of the home. The Gemara considers upper and lower stories of a home, and when a squatter must pay rent.  

We learn that the rabbis value any kind of damage done to a home because someone has lived in that home. They refer to "the blackening of the walls" of a home; the decrease in a home's value because it has been lived in.  However, if a person squats in a courtyard that was not meant to be rented out and the owner did not know about the squatter, rent is not owed.  Thus there is also a valuing of shelter for those who cannot find even basic shelter of their own.  Our daf ends with a conversation about consecrated beams or other parts of a property.  When an item is sacred, of course, that item must be treated with greater honour.

Does "blackening of the walls" refer to regular use of a home?  Is this a reference to stains from fire burning within the home?  Or something else; dirt from the floor?  Dirt from people's hands or clothing rubbing on the walls?