If the black part seems to move around, it may be a case of Poser Indecision
Syndrome.  When two surfaces share the same material, and they are in
the same place as seen from some angles, Poser will sometimes
get confused. 

I can't really tell from the picture, but you may have a plate just behind the screen,
both taking the same material?

Here's a case where I worked through the mystery:

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2704823

In that case the two surfaces were the top shelf and middle shelf of
the bookcase.

The solution is to give the competing surfaces different material zones
with different names.

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