Why are my goldfish turning black?

I went and bought some simple goldfish that were sold as feeders from petco a couple weeks ago and when i got them they started getting bacterial infections... I treated them with aquarium salt and they did great and are now healed completely but no i have another problem, they are starting to turn black!.. black patches keep showing up more and more almost if they were going through a color change there is only 1 outta 4 that is not doing this and it might just be because he is goldish black in the first place idk... i need some help...they aren't acting sick at-all they are spunky and are happier then when i got them but they just keep turning black.. first on the fins then now two have it on there foreheads..