The black point for the color space specified in CIE 1931 XYZ
This property is only valid for color spaces in which the
ColorSpaceType is CallGray, CalRGB
All components for this valud must be non-negative. The default
for this peroperty is 0 0 0.
When you set this property a clone of the XColor you assign is
created to avoid one XColor being shared by multiple color