Spray soil with fungicide at Lowe's Home Depot or fertilizer wholesaler depending on your location. My experience with growing container and raised bed bell peppers is that black and some times white mushroom or white specks on the stems of new seedlings are the result of my soil holding to much humidity. Through further analysis and corrections I discovered that I used soil mix that was too compacted , not enough drainages, conteiner holes were plugged by bottom rocks etc or the plant was kept in shaded area more than needed, etc. In my experience, once the soil caught mold I would not waste money on fungicide to correct issue as the plant already may have suffered root rut. I would just place that soil with plant and all in my compost to layer and get new soil with proper mix of 1/3 peat 1/3 couse vermiculite and 1/3 completely composted soil. The new plant in the new soil will make up for any lost time. Water is needed, but if the soil is too soggy your plant will just drowns unless is a lucky bamboo or similar.