You've got blossom end rot. It's caused by a calcium deficiency and exacerbated by irregular watering. Treat your plants with liquid calcium from the garden center for immediate results. Put crushed eggshells on the soil around your plants for more long-term soil improvement. Mulching your plants will help retain soil moisture between watering or rainfall. Mulch wont solve the blossom end rot like calcium will, but it can reduce the severity.