Black Dumpling can be Found in Volcano Manor.
The item is dropped by albanurics who have this same dumpling on their head. They can be mostly found lying around not very agressive NPCs
You can use either the Volcano Manor Site of Grace and use the first left door of the hallway (considering you have it unlocked) Or Guest Hall Site of Grace and use the location to farm it from these guys.
make sure to use silver tear scarab and silver tear mask to maximize the drop rate. Higher Arcane is also preferred