I've only been watering them. I have not used any food.

The leaves on some of the bell peppers have started to turn black.

Is this something I should be concerned about? Also should I be using some sort of food for these little guys?

At this stage of growth the plants have not yet developed the fibrous feeder roots they need. They only have what are called "water roots" (tap roots) so they are totally cotyledon-dependent for nutrients. If they are kept too moist those roots are stressed and the plant begins to develop what appears to be a phosphorous deficiency - thus the discoloring.

In my tests of them I have found that root riot plugs tend to stay too wet. It is difficult to maintain the proper moisture level with them.

To eliminate that problem pot them up into potting mix. I trim off as much of the cube as possible without doing any root damage. Use scissors. Then pot them up. That will trigger fibrous root development.

Once they are potted up and settled - give them a day or so - then a well-diluted to 1/4 strength liquid fertilizer feeding. Keep in mind that peppers prefer to dry out between waterings. They do not like to be constantly moist.

Hope this helps.

Dave

Thank you
Zach