We analyse the behaviour of black-body radiation in theories of electromagnetism which allow the electron charge and the fine structure constant to vary in space and time. We show that such theories can be expressed as relativistic generalizations of a conventional dielectric. By making the appropriate definition of the vector potential and associated gauge transformations, we can identify the equivalent of the electric and displacement fields, E and D, as well as the magnetic B and H fields. We study the impact of such dielectrics on the propagation of light in the so-called “BSBM” theory. We examine the form of simple cosmological solutions and conclude that no changes are created to the standard cosmological evolution of the temperature and energy-density of black-body radiation. Nonetheless the matter evolution changes and the behaviour of the entropy per baryon is modified, and the ratios of different dark matter components may be changed too.