Evolution with time of the source coincident with the position of AT 2021blu in the Sloan r-band. From 2006 to 2016 (top panels) the source is barely detected, with negligible magnitude changes. In late 2019 (bottom-left panel), the source is imaged by HST in the F606W filter, and it is about 1 mag brighter than in the decade before. LT images show AT 2021blu in outburst (in February 2021, approximately at maximum; bottom-middle panel), and at very late phases (late March 2022; bottom-right panel).