We present an approach to the design of mixed reality spaces that aims to create a more harmonized and unified user experience. We refer to these as blended spaces. Blended spaces draw upon a description of physical and digital spaces in terms of the ontology, topology, volatility and agency. By describing physical and digital spaces in these terms we are able to use the process and design principles of conceptual blending to arrive at a design that maximizes the relationships between the spaces. It also guides the development of the touch points between the physical and digital spaces. We discuss the user experience in blended spaces and briefly allude to the significant philosophical implications that blended spaces of the future will have to deal with.