The moment eggs are cut, bacteria begins to grow inside the egg. Some people cut that early but most dont. The black stuff is most likely just a build up of bacteria. If the babies are healthy otherwise they should be ok. Was there any sterile saline solution or bottled water added to keep the eggs filled? The more dry they become the worse the bacteria and mold can become. I wont get on anyone for cutting but I will recommend not cutting until after day 50 at least. I wait til first pip or day 54/55. Whichever comes first.