The first pair of pants I bought from Honig's when we made the switch about 10 years ago now looked to be more like a flaw in the white material of the stripe than fading.
Always wash in cold water, use better quality detergents. Dyes in synthetics usually don't run. Sweat (salt) however might be causing the black fading onto the white. Do you wear tights or just compression shorts? Tights or longer socks might give you enough of a barrier from skin to pant leg.