According to the CWCCA of America, blue merles can only be bred to tri's. Not reds, not sables. Just tri's and tri's alone. A Cardi could definitely work for you. They're a herding breed with touches of the primitive. They're watchful and vocal, but not typically outright aggressive. They definitely have cattle dog tendencies. If you have any interest in dog sports, be aware that a great many breeders focus on conformation and conformation alone. I'd look for someone who is actively pursuing performance titles with their dogs, can point to performance titled dogs they've produced, and is producing (relatively) moderate dogs - as in, dogs that aren't mostly big head, stuffy chest, and non-existant legs.