This is a Durban Poison, all healthy except one branch I just cut off. Looked like a dead spot in the branch so cut it just below that. Found what is probably just rot but I have not seen this before.
Bud rot is a type of mold that develops in the dense cores of cannabis buds. It starts on the stem inside of the bud and spreads outward, and it's difficult to detect in its early stages. It typically occurs on plants in the flowering stage.
I've never seen anything quite like that. Closest I've seen is when I've removed a side branch a little too brutally and the area became week. As Go Go kid rightly states, this is usually where bud rot comes from. That jet black though inside the stem - never seen that. I hope the rest of plant is ok - wishing you good luck!
Eh, this was less than 2 sq ft in a 9x5 tent so...could be a lot worse. Also, only grow for myself so I'm not losing out like somebody that sells it would be (not saying you do)