That actually looks like some shed skin was retained on the tail tip and cut off the blood supply as it dried.

This can lead to dying tissue which then comes off. I would try and keep it clean and see if it heals with the next few sheds - if it starts spreading or turning brown further up take it to a good reptile vet.

The feeding sounds fine - I often feed two smaller items if I think they are not quite ready for the next size up.