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Kirchhoff's law of thermal radiation states that for an object in thermodynamic equilibrium, its emissivity and absorptance (aka absorptivity) at any specific wavelength are the same. If an object absorbs 80% of incident light at some wavelength, reflecting the other 20%, then at that wavelength, its thermal emission is only 80% of that indicated by the black-body spectrum, i.e., 80% of that indicated by the Planck function. This explains why the blackbody concept specifies black: black means no reflection and if an object did reflect a percentage of some color rather than absorb it, then as Kirchhoff's law points out, its emission of that color's wavelengths would be reduced by the same percentage. It also explains why lightweight emergency blankets for surviving in severe cold are made reflective even though absorbing any incident EMR might seem to be helpful.