There's a rename command which can do what you want:

rename -n "s/\+\.png/.png/" blue*+.png

Remove the -n flag after testing to actualy apply the rename

On Debian and derivatives (including Ubuntu), there's a rename command which can do what you want:

rename -n "s/\+\.png/.png/" blue*+.png

Remove the -n flag after testing to actualy apply the rename.

This rename command is a Perl script, not to be confused with the rename command from the util-linux suite. Other distributions may provide it as prename or rename.pl or not at all.