The shadowplayers cels I've seen and own are just
black paint.  I think it's a form of special
effect to make it semi-transparent on screen. 
While I've seen shadows done as airbrushing on
"normal" cels, I've also seen lots of solid image
cels (full color too) where when on screen it
would be semi-transparent.  I think reflections
are often done this way too (solid in cel,
semi-transparent on screen).

Keys