It's site contamination. Anything feeding off the "veins" ,if you will, through the main stalk is getting contamination too, explains the discoloration and localization. I've seen this before in my own grow. Mine was caused from a wire I was using to train a branch outward. The stem, leaves, and bud where the wire touched discolored. After I took the wire off, the damage remained but the plant did fine. The contaminated bud was harsh and dry, smoked like shit but still got me high.
What I learned from that, never let metal touch the organics and always wash my hands before doing any work in the garden.
(edited) Side note: You're probably going to find that the triches on that bud are going to hit peak maturity, harvesting time, at a much earlier stage then the rest due to the stress.