What Causes Black Dot On Leg While Having Deep Vein Thrombosis?

Hi my mum had a black dot apsaras on her leg on the 7/1/15 then her whole leg started aching like mad and she has been diagnosed now with dvt. However no one has accounted for the black dot on her leg, I'm wondering if it could be skin cancer and that's why she has dvt. Just want a second opinion please.