The big bud of Lagerstroemia indica turns black.

Mainly water and sunshine. The buds are beginning to rot, which means you are ill. Do you use tap water? It is better to use other water. You should disinfect it if possible. Insufficient sunshine will increase the possibility of illness, because the resistance is low, hehe. Water should be changed frequently. If there is a dead layer on the bulb of narcissus, don't peel it.