A very nice name - more ancient than the Bulgarian state. It doesn't matter when or where it was first written down - people, places, and names don't come into existance from the moment they are written down. Humanity has a much older history than its written history.
Radomir is indeed an ancient Bulgarian name first known from the Medieval Bulgarian tzar Gavril Radomir son of Samuil (end of 10th century - beginning of 11th century AD).