Bladorthin was a King of Men in the Northeast sometime during the late Third Age. His realm was probably close in proximity to Lonely Mountain, because in The Hobbit Thorin and Balin remember and wonder if Bladorthin's order is still intact:
"The spears made for the armies of the great King Bladorthin (long since dead), each had a thrice-forged head and their shafts were inlaid with cunning gold, but they were never delivered or paid for."
Therefore Bladorthin probably ruled either Lake-town, Dale, maybe Rhûn, or Dorwinion. It says in the above quote that the spears were never delivered or paid for. This suggests that either the coming of Smaug or Bladorthin's own death delayed their delivery. The King's name appears to come from Elvish, most likely derived from blador, meaning "wide plain" and thin, meaning "grey". Actually, in the first draft of the Hobbit Gandalf the Grey was called Bladorthin. In the final copy, however, it the name only survives in this way.