I suspect the drop in the blue magnitude is due to the material cooling as the ejecta of the supernova expands. Taking a first approximation of a black body, then the shorter frequency blue light will reduce first as the explosion cools. You can see the effect in Robin’s spectra animation from a few days ago.
With the caveat that I am not an expert on supernova, so someone else may be able to jump in to confirm or refute this notion.
Andy
British Astronomical Association
Supporting amateur astronomers since 1890
