We had some very intense sunny days when these started developing peppers, they may be sun scorched. Thanks for the article, I'm growing cayenne peppers for the first time this year so it was an interesting read.
What?!? Corking is how you know they're good and hot. As they mature, you cut that water back and give them long dry phases to up the heat. Makes them even more delicious!
The black is fine, I think. Usually blossom end rot or other bad stuff looks kinda swollen and yellow/tan colored, in my experience. Black usually means they're ripening. It looks black now, but it's actually red and green pigments at the same time. As the ripening progresses, the chlorophyll goes away leaving red chilis.
Looks like perfectly normal anthocyanin development. Anthocyanin gives peppers a purple color, particularly when unripe, and is high in chlorophyll. Not anything to worry about, and in fact it just means that they're getting plenty of sun and nutrients.