When Captain Blood and the pirates first sail the sky behind the ship looks odd and full of bumps. This is because it was a backdrop painted on the poured concrete side of a sound stage which was built on the edge of the pool.
Though the film is set in 1690, one of the characters sings "Largo al factotum" from Rossini's opera "The Barber of Seville," which wasn't written until 1816.