This is totally normal. I don't know the reason although I suspect it is caused by a temperature change. Many varieties of peppers will do this but I think jalapenos are the most common and bananas next. Don't worry, it doesn't hurt a thing except its looks and taste buds don't care what something looks like.
There is nothing wrong with the pepper at all. All the black is is a skin blemish. Don't worry about it. It will taste just the same as without the blemish. Please update your profile so we will all know where you live and have an idea of what could be wrong.