Looks like it could be bruise or possibly sunburn? Did it fall or anything? You should check the base of the stem and see if there's anything suspicious there.

Bob,
Those look like the three I sent you back in Sept. 2012. That blemish may not be that noticeable once they get 3 ft. tall. If it gets worse, I got more yet.
Stush

Stush,
These are the ones you sent me. At the rate they're growing, three feet tall isn't very far away. Thanks again for the plants.

David,
There was a storm and I remember a few plants being blown over. I can't say I remember which ones thou.