A blackened stem indicates there's too much water. Probably the soil you're using is not drained enough and/or you are watering them too much. Peppers need well drained soil and plenty of sun. They don't mind a day or two without water. In North Italy the best pepper-growing areas are where gravel is plentiful in the soil, such as near rivers.
I would start by decreasing water and also consider adding sand to the soil if your soil is rich and tends to retain moisture.
Hope this helps. Keep us posted!