Black-body fits for AT 2021biy with the multi-band observed data. Left: SED evolution of AT 2021biy; for each SED, the best-fit single BB function is also shown. The +102.9 d SED extended to the MIR thanks to the WISE data is shown in the inset. Blue lines represent the fits built with the optical data, while red lines are the fits based on the optical and NIR data. Right: BB parameters of AT 2021biy, AT 2017jfs, AT 2018hso, and V838 Mon. Top-right: bolometric light curve; Middle-right: BB temperature evolution; Bottom-right: BB radius evolution.