Sounds like a revival of the Bushhamster!! He talked so much about his purple reciever we all came to love it. Great fun.
Fight4 is right. Black is rarely true black in anodizing dye. It's a very dark blue or sometimes green. When the tank is fresh, it looks black. When the dye tank needs replenishment, the color starts to show. No functional problem, purely cosmetic. Lucky you, you have a unique rifle!!