It is commonplace for jalapeños to turn black during the growing process. The color of the pepper is subject to change the longer it remains on the plant.
Jalapeño peppers vary in size, texture, taste and color. Most jalapeños are harvested when they are bright green. However, if they are left on the plant, the color continues to change. The bright green color slowly starts to grow darker until it turns black. The black color signals the last phase of the transformation of the jalapeño pepper. Red jalapeño peppers tend to be sweeter and not as spicy as the traditional green pepper.