Wash in Synthrapol in ***lots*** of hot water, and it should come out. Might take several washings, though. You really need a top-loading washer or the kind of front-loader that laundromats have. Synthrapol will usually work even if the fabric has been dried, as I believe most modern commercial dyes are not heat-set at normal wash/dry temperatures.
By any chance did you wash it in a domestic front loader? There is a lot more risk of this happening in that kind of washing machine because of the small amount of water used in the load. Usually the dye is simply excess dye that was not properly rinsed out of the fabric. When washed in a lot of water, the excess dye is sufficiently dispersed that it does not create a bleed in other fabric. There is much more opportunity for bleeds and "crocking" of dye (dye that transfers via rubbing, especially when wet) when there is not enough water to disperse bleeds. I have found that *all* of my daughter's black leggings bleed a little every time they are washed in my front loader, so now I routinely keep colors separate as much as possible and also use color catchers in almost every load.