WMG / The Three Caballeros

Jose Carioca and Merlin have met

... and it is specifically because of knowing Jose that Merlin is so against using black magic. "Never touch the stuff!" There is also a good chance Merlin ran into Jose while visiting 20th century Bermuda, and was part of the reason Merlin returned to medieval England. Even for Merlin with all his "hockety pockety," the three Caballeros are just too trippy and surreal and loud for him to tolerate, and their black magic gives him a headache.