Captain Blackheart was an anthropomorphic cat.
Peg-legged, eyepatch-wearing Captain Blackheart was the captain of a pirate ship in the second half of the 17th century. By 1687, he and his crew had created a primitive settlement in Duckburg Bay, out of sight of Fort Drakeborough. That year, Blackheart encountered a time-traveling Scrooge McDuck, his grandnephews, Gyro Gearloose, and their enemies the Beagle Boys — all, coincidentally, on Blackheart's birthday.
Revealing himself to be a rather genial sort of pirate, the Captain only took the gang prisoners so he could make them sing at his birthday party, at which he was offered a single new boot from his sailors, whom he thanked profusely. He did, of course, plan to take the time-displaced fortune of Scrooge for himself, but that plan fell through when the Ducks, theBeagles and Gearloose took it back to their own time-period in their escape.
Behind the scenes
Captain Blackheart resembles Peg-Leg Pete down to the peg-leg, only differing by his attire and eyepatch. Along with Captain Pietro and Genghis Khan, he is one of three seemingly-unrelated characters in DuckTales modeled after Pete. Though the same cannot be said for the others, it is possible that Blackheart was one of Pete's ancestor, somewhat excusing the puzzling resemblance.
Coincidentally, Captain Blackheart shares his name with an undead pirate from the Elder Scrolls Online video game.