Here's another photo of another pepper.

Kinda looks like sunburn to me. I have it on some of mine too, the more experienced growers can tell you exactly what it is though.
Randal

Correct about the sunburn -- a mild case. The pepper will be fine.

Okay, thank you very much guys. I appreciate it.
What can I do to prevent it from worsening? It only gets afternoon sun as it is on the back patio.

You COULD shade it even more but I wouldn't worry about it since it's only a cosmetic issue and usually only affects a fruit or 3. Rotate the container maybe.

Kevin

Thanks for the replies guys, I was fretting thinking my peppers were diseased or something.

I appreciate it.

As they said above. When it ripens it will look normal as well.

Jay

Yep, sunburn. I have peppers that have much more of the discoloration. My Aji Limon are really sunburned but they tend to lose the discoloration when they ripen. I get it every year on some of my peppers and it never affects them negatively. I live in Wisconsin, so I would expect it to be worse down south. Nothing to worry about at all.
Bruce

This may be one of my problems too. Growing in philly so not to far from your garden.