Charles Barkhouse, an Oak Island historian and family friend, described Mr. Blankenship as a “living legend.” “I mean, how often do you get to meet a treasure hunter?” Mr. Barkhouse chuckled. “In truth he was much more than just a treasure hunter. He had a very full life.” Mr. Barkhouse said Mr. Blankenship was a U.S. Army veteran who had a successful contracting business in Miami, Fla., when he got hooked on the Oak Island mystery after reading a Reader’s Digest story in 1965. “He’s poured his blood, sweat and tears into that island trying to solve this mystery,” Mr....