Add canned tomatoes, chipotles in adobo, chopped onion, minced garlic, cumin, Mexican oregano, and salt to a blender. Blend on medium speed until fairly smooth, about 30 seconds to 1 minute.
Pour the contents of the blender into a 4-quart slow cooker. Add the chicken breast, fully submerging it in the sauce.
Cover and cook on LOW for 4 to 6 hours, or until the internal temperature of the chicken is about 160 degrees F.
Remove the chicken from the slow cooker and shred. Return shredded chicken to the slow cooker and stir it into the sauce. Cover and keep on the warm setting until ready to serve.
Serve hot on tostadas or in tacos, quesadillas, enchiladas, or burritos. Alternatively, let cool and store in an air-tight container in the refrigerator for up to 5 days.