I don't like how the bottom of that toe looks. It is very dark like the tissue is dying. That could indicate that the toe has become infected. It may have gotten caught on something or bitten by a bug of something like that. It's hard to say for sure what the cause was. Since it is so dark and looks to be withering a bit, I would try applying a topical antibiotic cream like neosporin (plain, without pain reliever). If the dark area appears to move further up the hand, then that may indicate that infection is spreading and a vet will need to treat it. I would probably seek the opinion of a different vet before returning to the last one you visited, if possible. Can you post some more photos of the toe from different angles?