Peppers are turning blackish purple then to red
Yep! They are just turning ripe! Most people pick them earlier on to prevent them from falling from the plant and rotting. They tend to do so when ripening. They can be just a little spicier when picked later, as well.
This article will offer more information on Jalapeno care:
https://www.gardeningknowhow.com/edible/vegetables/pepper/growing-jalapeno-peppers.htm