It is a normal occurance in a lot of plants. The darkening in nodes is more pronounced
in some variety of peppers than others. To me, a node has a complex structure, as we see in a knot,
how a branch of a tree
grows out of the main. It has much higher density, strenghth and tight grain pattern.
The same principal applies to peppers, which in tropics can live and grow for years.

I am sure you will get better explanation than mine.

Agree - leaf and branch node darkening is normal for many varieties of peppers and other woody stem plants too. As the core of the woody stem ages it darkens. And it seems that the amount of wind exposure speeds that up at the branch nodes as the fibers stretch to compensate for the wind.

Do you have it on the fruit itself? That is something else. Perhaps some BER (blossom end rot).

Dave