Nute lockout or burn, or your soil has gone sour. Did you put peat in it when you planted? You've got crispy tips and a purple plant which makes me curious what your overnight low temps have been, and if you've cut back watering and fertilizing yet.
The purple bud may just be genetics; some strains go nearly black when temps dip into the 50s or even low 60s overnight. I don't see rot on the buds themselves. That's more like sections of the bud start to look grey and either slimy or fuzzy. Yours look healthy.
However I see what looks like mold on some of the sugar leaves. That is not good, and should be removed from the plant immediately. Look where you see white fuzz over dark patches. Google 'powdery mildew' for verification.
If it spreads to the buds, you're in trouble. And if it spreads to the other plants it's bigger trouble. You're close enough to harvest that you might just want to take that one down early for safety, and dry it in a location that you won't use for the rest of the plants later.
The leaves look scorched. The combination of types of damage shown are consistent with cold, wet, acidic soil. This is common when growing in peat based soil in cool temps. No more fertilizer, and go easy on watering as it gets cooler and growth slows.
Good luck!
Edit: plant in third pic is too far from harvest to stop feeding. Light flower nutes, foliar spray with GHMicro on whatever you can hit without wetting the buds, your may need to pH adjust your solution but let us know about the peat situation first.