convert blackbackground.png -fill white -opaque black whitebackground.png
convert bluebackground.png -fill white -opaque \#200070 whitebackground.png

http://www.imagemagick.org/Usage/color/#replace

When you put an image in a printable document, you don't want it to have black (or coloured) background.  ImageMagick allows to replace colours even when the coloured regions are non-contiguous.  Arbitrary colour values can be specified with `\#123456` (the backslash is for escaping the #).  If you want to replace similar colours as well, use the `-fuzz` option.

Replace background colour

replace, color

Bash