I'm a big Gordon Bok fan. But his frequent partners, Ed Trickett and Ann Mayo Muir, have to be among the blandest singers I've ever heard.
Margaret MacArthur is another singer who is much too bland for my taste.

As for Dylan, I'm a huge fan of his stuff dating from about 1961-1976 and from about 1990 to now. I didn't like his stuff from the late seventies and through the eighties.

Personalities that have turned me off their music are Don McLean and Michael Johnathan.