What Causes Dark Line In Between Ankle And Foot?

I have a long dark line on the from of my foot. It starts up by my ankel and runs down about three inches on my right foot and its also on my left foot, about 2.5 inches. This is on the top of my feet. They loom a brownish/purple color. Any idea what it can be? Its nottender or anything and I haven t had any new shoes or worn any more than one day at a time, so not the same shoes every day. I m at a loss...