To secure a document, it is very important to use multiple security features.

A document that contains only one security feature, a VOID background for example, may be easily duplicated on some photo copiers without the word VOID showing up on the copy.
However - add a ghost watermark to the back of the sheet, color change ink and micro printing to the front of the sheet; now you have created a document that is virtually impossible to accurately duplicate using a photo copier or scanner.

SECURITY TECHNOLOGIES AVAILABLE
from Micro Format, Inc.
for use on custom printed DOCUMENT SECURITY PAPER.
20 different illustrated features
both "overt and covert" features

While Document Security Paper offers protection against unauthorized duplication, the image can still be copied or scanned. The text or graphics will be visible on the copy and can be read.
However it will be possible to determine which document is the original and which is the duplicate.

As far as today's technology goes, other than dark red paper or paper printed with a dark purple background, we do not know of a product produced anywhere in the world that will cause a photocopy to turn black (and this happens only on a black and white copier when a document has been printed on Dark Red or Purple Paper).
Even red paper or paper with a dark purple background can be duplicated when using a color copier!