They is such a color combination. I use to ride a tobiano blue roan Arab cross pony. Flashy devil! He was handsome.
However, breeding for one isn't a good idea. I've seen very, very few; they aren't rare from lack of trying. If you want to attempt this cross, make sure you have something going for the foal if he isn't colored: conformation, bloodlines, training, and all that. Spots don't make badly bred horses any better.
Or you could just buy a blue roan Paint. It's easier that way.