Photometric evolution of AT 2021blu. Top panel: long-term light curves in all filters. Transformation relations from Jester et al. (2005) are used to convert Sloan u-band photometry to Johnson-Bessell U. The solid lines on the left represent the magnitudes of the quiescent progenitor of AT 2021blu. The down arrows represent upper detection limits. The dot-dashed lines represent the uncertainties of the progenitor detections in the different bands (see Table 1). Bottom-left panel: close-up view of the peak of the outburst in the UV bands. Bottom-right panel: detail of the LRN light curves in the optical and NIR bands from −75 d to +170 d from the g-band peak (MJD = 59258.89). Data from the public surveys are also included.