The Chosen Ones

You have to complete the quest "Secret meeting" where 4 citizens must be found and return back to the home, where the quest-giver stays. After it, the time won't be frozen on midday. Therefore - it is not a bug, and black merchant is located near of the Lighting Esper location - on north-west from it, the item he suggests to buy is the last item of the Demon Sphere set.