Digital BW, The Print

BTW for those (few ;-) ) that are interested, the short white paper by Adobe
on Black Point Compensation is located here:

http://www.color.org/Adobe1bpc.pdf

The maths is at the very end, in section 6 where the formula for "Black
Point Stage" is given.  I am not good with maths but (having had Carl
Schofield remind me twice) I do know that the formula for DecodeLab(L) is
simply XYZ_Y.

(Carl provided me with L*x27/24389 for 0<L*<8 which is the same.)

So the Black Point Stage is a diagonal matrix containing the ratio:

Scale = (1-Yd)/(1-Ys)

Where Yd is the XYZ_Y of the destination black and Ys is the XYZ_Y of the
source black.  

Simplifying the statement to XYZ_Y is where my comprehension ends.  Unlike
Roy I have not had to apply that formula.  It talks of a diagonal matrix
which I think for our case is not relevant in that we are only scaling
luminance.  

Time to head out for dinner...