MRTD

Surprisingly, Minimum Resolvable Temperature Difference (MRTD) measurements can be made with an accuracy better than the "total system uncertainty" spec of the blackbody. For small incremental temperature measurements made near zero DT and spaced closely in time, measurement accuracy is determined by the temperature noise (short term stability) and linearity of the blackbody, rather than its absolute accuracy.
Recall that MRTD measurements are made by recording the minimum resolvable positive contrast, the minimum resolvable negative contrast, and then averaging the two. This removes any inaccuracies in the reference (target) temperature. Additionally, the MRTD measurement now depends not on the accuracy of the blackbody, but on its linearity (that is, any offset error in the blackbody temperature will eliminated the positive and negative temperatures are averaged. The only remaining error is linearity: how well the slope of the temperature calibration curve matches true temperature).