That radiation becomes black-body radiation through thermalization processes (such as photon absorption and emission interacting with matter) is a foundational principle of statistical mechanics and thermodynamics, requiring no citation.
The claim states a textbook definition from thermodynamics and statistical mechanics: thermalization describes the process whereby radiation interacts with matter (via absorption, emission, or scattering) to reach thermal equilibrium, yielding a Planck (black-body) spectrum. Because this is a definitional truism of physics, it is classified as common knowledge and requires no citation.