What are they and where did those dark lines around the baseboards come from?

The fine soiling of Filtration Lines!

Filtration lines are what they are called.
        You first notice the edges of your carpet getting  a little dirty, it happens slowly and then one day you realize that they are really dirty, and it just kinda crept up on you. Sometimes in months but most likely years. This has nothing to do with the carpet itself but the pollutants in your homes air, and the air movement along the walls, through any flooring cracks, or joints, and wall cracks. Also movement from the upstairs to the downstairs. Filtration soiled areas are most likely to appear around the edges of a room up against the baseboards, underneath closed doors, or where the doors stay in one position for a long time period, and along the edges of your stair steps. Your stairs are often the worst of it, as the concentration of air flow is normally from upstairs to the downstairs areas. Often when you have them in the above listed areas, you may see soiling in some open areas too. Mostly along the seams where the joints of your sheets of plywood sub flooring are located.    
        Filtration soiling can be very difficult to remove, due to the fact that it is made up of very fine particles
that have been allowed to build up in the carpet pile, or actually filtered by the carpet pile for a long period of time. They have most likely been whetted from previous carpet cleanings (but not removed) and then allowed to dry, making for a big reason for them being very difficult to remove. Just like a setting in stain by putting a wet shirt in the dryer. These filtration line particles can be made up of many items, including fireplace residue, residue from smoking inside the home, automobile emissions, (live near an interstate?), residue from polishes and cleaners, cooking oils, paints fumes, burning candles and incense sticks, and numerous other indoor and outdoor pollutants. All transferred around the interior of your home by your Hvac system.
        This type soiling is virtually impossible to remove by the home owner, and you should call a professional. After cleaning, make sure a commercial grade of Scotch guard is applied, and the preventative steps below are taken.

Filtration Line Prevention:
        1.  Get your Hvac vents and ductwork system cleaned, and remember to change your filters 
             more often than required if your home is susceptible to filtration lines.
        2.  Reduce or stop any personal Smoking, or smoke produced by fires in the fireplace, and 
             candles and incense burning.
        3.  Vacuum twice weekly if your home is subject to Filtration soiling. Use the edge tool or      
             crevice tool to clean the edges of the room and the areas where you know that filtration 
             soiling has previously occurred. 

Hopefully your vents are not this bad, but you should definitely have them cleaned if you have developed filtration lines.