Well, could be something in the water specs that is irritating. I'd check the whole lot just in case - ph, ammonia, nitrite, etc. I assume a liquid test?
Is the flashing constant or more at meals or after a water change? Any pattern? Nothing else with the fish, are fins all normal? No marks, red dots or bumps on scales? Breathing and swimming normal to other fish?
Prime should take care of any heavy metals, etc and your other fish seem fine. I've never heard of a bad reaction to it.
Have you checked your tap (?) water that you use for changes for ph, ammonia, etc? Assuming the tap water is fine, I'd increase the pwc
to 40-50% which in general will help and keep ammonia in check.
It does seem odd, if the pet store had this fish in blue-coloured water, was the water different to other fish tanks? Could they have been treating the fish?
Next up I'd look at a salt treatment or salt bath in a separate tank which also shouldn't make the problem worse but could help if there are parasites in the gills.