Black body radiation is the maximal possible thermal radiation ($\varepsilon = 1$). Real bodies are not perfect BBs, so the most primitive factor taking this into account is $\varepsilon \le 1$. It a factor (multiplier) at the specrtal curve.
I am guessing it is the emissivity https://en.wikipedia.org/wiki/Emissivity which is a factor that you multiply by the curve because things are not perfect blackbodies, depending on what the surface is made of.
This video by Prof. M. R. Shenoy beautifully explains everything related to resolving powers and limits of telescopes and microscopes.
Your doubt is explained at around T$=20:56$ .
This screenshot/diagram should clear everything up.