The stem on one of my T. Scorps and a bell pepper plant have this long brown stripe running down it. I did a quick google search but couldn't find anything.
Theres nothing wrong with your plants. When your plants start to mature all of them will get a darker stem from getting stronger and more sturdy. Its called harding them off
if its brown i would keep an eye on it. i don't want any brown on my plants i start to assume blight, fungus or virus. i cant tell if its brown or purple but if purpl is ok.
good luck take a better pic if you can will help