Blau space consists of the multidimensional coordinate system, created by considering the set of socio-demographic variables as dimensions. All socio-demographic characteristics are potential elements of Blau space, including continuous characteristics such as age, years of education, income, occupational prestige, geographic location, and so forth. In addition, categorical measures of socio-demographic characteristics such as race, sex, religion, birthplace, and others are Blau dimensions. "Blau space" is a theoretical construct, named after Peter Blau, first presented in McPherson's "Ecology of Affiliation" in American Sociological Review Vol. 48 (1983), pp. 519-532. However, a more sophisticated version is presented in McPherson and Ranger-Moore's "Evolution on a Dancing Landscape: Organizations and Networks in Dynamic Blau Space" in Social Forces Vol. 70 (1991), pp. 19−42.
The organizing force in Blau space is the homophily principle, which argues that the flow of information from person to person is a declining function of distance in Blau space. Persons located at great distance in Blau space are very unlikely to interact, which creates the conditions for social differences in any characteristic that is transmitted through social communication. The homophily principle thus localizes communication in Blau space, leading to the development of social niches for human activity and social organization.
Although the idea of Blau Space was originally developed to consider individuals, it was extended to the context of cities (and other urban units) in Neal's "Building a Blauürban Space" presented at the (Midwest Sociological Society) in (2006).