If it's not botrytis it will be. High humidity, cooler weather, poor circulation are all factors. Cut that bud off and check for bud worms, sometimes they can cause it. You should cut that bud off because if it does turn to botrytis it will infect the whole plant later.
this is budrot. looks like yours so im saying yes. but does kinda look burnt? if its soft and squishy (not burnt and crispy lol) then its definately bud rot. but if it crumbles from being so crispy its just burnt.
and whats humidity? humidity during flowering supposed to be 35%-45%