I believe Blue Roan Tuxedo's do exist. Meet one of our Working Cocker Spaniel mums, she's called Molly and her mother was a blue roan cocker.
She had just had a litter of varied coloured F1 cockapoo puppies but they include two chocolate roan pups from an apricot/white parti poodle dad. So that roan gene has to have come from Molly I think.