> the edge of the image.  There is a border of 
> original paint pretty much all the way around.
> So if it was backlit, the edges of the
> characters would "glow" (unless that is part
> of the effect!). 

No, the black layer doesn't have to match exactly
the picture : the paint is not transparant even
when using backlight. The black paint is used
mostly to prevent some part of the cels to look
lighter and other darker because the paint is
not everywhere the same.

> thought the sword would be done with a 
> backlighting effect, but on this particular
> cel, the area where the sword blade should be
> is painted black 

Most glowing effects are not done just by using
light. They are added with a computer once the
cels are digitalized.
It's normal that the sword is painted in black :
that's a chroma mask. It's then easier to just
replace this area by whatever effect you want
with a computer.

Cheers,
Yann Stettler