Returns the CLUT index to produce black at the current screen depth,
assuming a standard color lookup table for that depth. E.g.
When the screen is 1 to 8 bit mode, the Macintosh OS always makes the
first clut element white and the last black. In 16 or 32 bit mode the
clut goes from black to white. These CLUT conventions can be overridden
by Screen ‘SetClut’, which makes a direct call to the video driver,
bypassing the Mac OS, allowing you to impose any CLUT whatsoever.