Why is a purple Martin called a purple Martin?

Purple martins were initially though to be closely related to European House Martins due to their forked tail, but they are only distantly related, is one though. Another theory is that they were named after the Roman God of War "Mars" and the "tin" or "ten" is a pet name...so basically "little mars" either due to their tenacity in chasing off predators or perhaps as Mars refers to the first month of the calendar, when the first martins start to return.

The Purple part is a misnomer since they are really a deep blue-not purple.