  The blackbody theory is revisited in the case of thermal electromagnetic
fields inside uniaxial anisotropic media in thermal equilibrium with a heat
bath. When these media are hyperbolic, we show that the spectral energy density
of these fields radically differs from that predicted by Planck's blackbody
theory. We demonstrate that the maximum of their spectral energy density is
shifted towards frequencies smaller than Wien's frequency making these media
apparently colder. Finally, we derive Stefan-Boltzmann's law for hyperbolic
media which becomes a quadratic function of the heat bath temperature.
