By the integration over an exponentially expanding universe of the fluke electromagnetic radiation created locally. Maybe matter is created as neutrons that decays into hydrogen atoms which would then radiate something.
Photons emitted from radioactive decays have specific wavelengths. Even with Doppler broadening blurring it out, I don’t see how integrating over all decays at all distances would produce a black body spectrum.
The black body spectrum shape is actually really hard to produce through another mechanism. In fact, before the discovery of quantum mechanics attempts to calculate a black body spectrum with classical mechanics failed at short wavelengths. This problem was called the ultraviolet catastrophe.
Maybe this is what would falsify my proposal… unless maybe that fluke local radiation could start at the maximum energy of the black body curve ? I will try to look into this.