What Is That Black Around the Edges of My Carpet?

This is called filtration edges. It is caused by an air gap between the wall and the sub-floor. The air movement along with the dust creates a static charge to the dust particles and binds to the carpet. They can be permanent. The best way to avoid them is to seal any cracks/gaps before new carpeting is installed.

Sometimes the filtration edges can be removed depending on how long they have been there.