Dark coloration like this on the stems of your pepper plants are not typically anything to be concerned about.  Sometimes a dark color forms on the stems of peppers as a response to certain environmental conditions or stresses.  Other varieties are just more likely to produce them. They do not affect the overall health of the plant or the developing fruit.  No action is needed.

If the dark colored spots on the stems become soft and mushy or if you are noticing stem or leaf die back around these areas, then that is more likely a disease issue.  That is not what this appears to be.  I recommend that you keep an eye on it to note if anything changes, but as long as the stems are firm and healthy, the dark coloration is nothing to worry about.

Happy gardening!