Hi
as goldfish dont turn black then i suspect what you are seeing could be melanophore migration. The pigment in the skin darkens in reaction to ammonia/nitrite burning and is because at some stage you had a spike of one or the other or both. maybe a mini cycle. The blackening is the skin healing and if it is the culprit then they should start to get healthy again in a few weeks. Keep testing the water for ammonia and nitrites. If your test kit is more than 6 months old then you should get new ones. Hope that helped.