Pepper growing question.
 

I know this question isnt regarding a hydroponic system but maybe some one may know the answer since I am very new to growing vegetables.

I have 4 different types of pepper plants growing in my garden. All of them have recently started getting a blackish, brownish tinting in their stalks. I have incldued some pictures. It is hard to see but look at the stems.

What is this? Is it normal? Or should I be worried? What steps should I take to correct it if it is bad?