Inverted ColorKey Blits

Hullo there,
I have a sprite that I want to blit in all white (when the player takes
damage etc.). This can easily be done if it is possible to blit the
inverse of the regular colorkey blit (namely all the pixels having the
colorkey color instead of the rest). This way you would also be able to
copy a colorkey mask from one surface to another.
I have copied the SDL_blit_K.c and made negations to all the checks in
order to get it work locally, but isn’t it a feature that would be good
to have in general, or is there another way to do this that I have

Yours sincerely,
/morten bek