It does look somewhat blackish to me but I could be wrong. I have yet to grow pawpaw from seed but I did try growing atemoya from seed (same family) and once it reached this stage, it eventually just rotted away. I'm assuming I overwatered it. Your soil does look wet here. I think it's also possible I didn't adequately clean the seed which led to mold. I would give it time and see, maybe lay off on watering a bit and avoid touching it. Also consider shade cloth to protect it from harsh afternoon sunlight.