Why are my jalapeno peppers turn black?

It is just the type or variety of the pepper. I planted some green bell peppers and had some of them that were a bluish black color. There are literaly hundreds of varieties of each kind of peppers just like people. There is nothing wrong with your peppers. But different colors mean slightly different flavors.

Jalapeno Peppers Turning Black

You are leaving them on the plant too long. Next they will be red, but jalapenos are picked green, because when red they usually have bugs inside.

This is natural as they mature. If you leave them on the plant, they will also begin to turn red