This is complete nonsense. First of all, farmed Discus are raised without any substrate at all. Secondly, wild Discus inhabit forested areas that are devoid of sand. Although sand substrates do exist in some areas, those are the exception not the rule. Most substrates in the Amazon basin consist of mud and decaying plant matter. These substrates are closer in color to black gravel than than they are to play sand.
In my earlier years, black gravel was my substrate of choice. I kept several species of both African and South American species during those years. I never experienced what you have described.
It's clear the OP's Discus is experiencing stress, but it's absolutely not due to the color of the substrate. The OP doesn't mention what other inhabitants occupy the aquarium or the water parameters. Nor does the OP mention when the fish was purchased, whether it was quarantined, or when it was transferred to the aquarium. If it's been recently added it's likely still acclimating. It may have been carrying parasites or infected with a disease at time of purchase. It's also possible the lack of background is a source of disturbing reflections, or the lighting is too bright.
In short, there are a host of reasons, all of which are more likely sources of stress than the color of the substrate.