How do you get rid of the black gunk in carpet on the edge of stairs?

What causes it?  The carpeting going up my stairs is starting to turn black on the edges.  I bought a home once before that had these stains and never could get them out.  How do I clean it before it gets too bad and what causes it?

Those grey to black areas on carpet edges are a result of moisture and soap residue left from past cleanings. The residual moisture attracts and holds settling dust. In the past I have had some success with a carpet steamer, hot water and extraction but you will have some slight discoloration.

Hope this helps