Sena

The deep bedding. Superworms packed into deep substrate generate their own heat, so the middle of the tub ends up much warmer than the room. Keep it shallow and spread them across more containers rather than one deep one. Cold killed these; crowding would have been next.

The fridge. That is the whole answer and it is the single most common mistake with these. Mealworms tolerate being chilled to slow them down; superworms do not, and cold kills them. A superworm that dies darkens quickly, which is why your tub looks like a colour problem rather than a temperature one.

Get them out of there and onto a shelf at room temperature, somewhere in the low twenties, and stop refrigerating them entirely.