Joey Bloggs was a miner in the colony of Paradiso, mentioned by Sheriff Bourne when interrogating Malcolm Reynolds and Zoe Washburne about a theft from the train they were riding. Little is revealed about Bloggs, save that he took his own life several weeks prior to the events of "The Train Job".
It's possible that Sheriff Bourne simply made Joey Bloggs up on the spot as a test to catch Reynolds and Washburn in a lie. "Joe Bloggs" is the United Kingdom's equivalent of the North American "John Doe", and is used in the same manner.