are you talking about the dark colouration marking individual scales around the gill area, or the frilly dark bit within the gills? the frilly bit is normal, that's what they flare and display with. the colouration on the scales I couldn't see in the original pictures. could be a result of stress caused by a ammonia or nitrite peak, could even be ph related. until you get the test kits I would recommend cutting back on feeding which will slow any ammonia, nitrite build ups. some fish will swim funny with a ammonia peak, fins clamped and close to body almost like a slither/shimmy action.
I am glad to hear he is still ok, hopefully once you get your test kits you will have a better idea of what's going on in your fish's water condition.