Optical spectra of AT 2021blu, corrected for the redshift (z = 0.002098) and the reddening (E(B − V)_MW = 0.02 mag). Top panel: spectra from −7 d to +30 d. Bottom panel: spectra from +32 d to +146 d. A few spectra are quite noisy, and a poor S/N spectrum at +16.3 d is not shown. Residual fringing is visible in the FLOYDS spectra. The instruments and the phases from the g-band maximum are also indicated.