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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.

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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.

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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.

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