[Edit] hmm.. actually, the catcher material does not seem to become completely black.
I had to hack it ;)
So it's not a bug that it's not completely black... mmhh... strange (not really sure if it's really useful then).
you can use the "visible for reflections/refractions/... to disable the visibility of the material. At the moment I am not sure how to best integrate it into UI
What exactly is the catcher property "meant" to be used for? I mean, I see it as a quick and dirty way to give people the opportunity to get shadows of their car renderings on a background - but it is not comparable to real matte/shadow stuff.
So what exactly do you want to integrate in a UI?