What you mean by gone black? Got a picture? Is it the hairs or the buds them self?. Normally the white hairs will turn brown and 4 weeks sounds about the time there start to turn.
What is the ambient air temperature? Buds/leaves/stems tend to turn dark purple in cold climate.. thats all i can think of..
I have an outdoor patch with really darker than purple buds on it, almost black. Its a deficiency of some type of nutrient .. or to high of a nutrient. I dont know how to help this outdoors cause I cant flush it if Im wrong, I leave em be.
if you are in a really cold climate its likely the temperature turning the buds this dark colour. If you are in a very humid climate its probs mold. Bud mold will generally not turn dark until it has been affecting the plant for a long time.
A good way to check.....give the bud a few gentle tugs with your fingers. If it is mold the bud will break away easily and stick to your finger. If its not mold the bud will be robust, stay intact and remain firmly attached to the plant.