Quote - ...50% gray in Poser is NOT neutral.

True, but I can use grey as neutral if I manipulate the map with math nodes to both expand the range and re-center the range of values.

Quote - So, if black is neutral and white raises the surface in Poser, how does one create a a displacement depression? There is no blacker than black, is there?

There is! ^^ 
The original map consisted of shades ranging from black (=0), through grey (=1/2) to white (=1).
I used math nodes to spread the range of values and to re-center it;  after manipulation, the values now range from -1 ("blacker than black"), center at 0 (black), and continue to +1 (white).
The negatives engrave, zero leaves the surface unchanged, and the positives raise detail.

If on a given material you only need to engrave -with no need for simultaneously raising  detail- then it is easier to simply plug in your displacement map, but set the displacement value to be negative.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5