As an aside, there seems to a misconception running through Davdan's thought, about how a "black body" spectrum arises.
It may be of wider interest to clarify the process. A black body spectrum is emitted by matter which is at thermal equilibrium. The model of a "cavity" is used so as to get the radiation enclosed within to hit the walls, (which are made of matter), and be absorbed and re-emitted, until the matter in the walls and the radiation reach a mutual equilibrium, at which the amount of radiation emitted by the walls and the amount absorbed by them is the same. The sole purpose of the "cavity" model is to provide one simple scenario in which this state of affairs can be reached. But there are others, notably the plasma in the photosphere of a star.
In a plasma, electrons are ionised from their parent atoms, which means they can absorb and re-emit photons of any wavelength, so you get a spectrum which is a continuum, rather than being restricted to the spectral "lines" you get when the electrons are in bound states in the atoms. This means that, if you have radiation being continually emitted and absorbed by a plasma, the spectrum of the radiation will be determined solely by the energy distribution of the ions and electrons comprising it. That energy distribution, which is determined by statistical thermodynamics, is characteristic of the temperature of the plasma.
This is what a black body spectrum is.
You do not need a cavity. You do not need reflection.
What you need is matter that is free to absorb and re-emit any wavelength, in equilibrium with the radiation thus emitted and absorbed.