Very true, but I wonder if other aspects are involved here.
When converting from 0C 0M 0Y 100K (US Sheetfed Coated v2) to one of the "dot gain" greyscale profiles in Photoshop, I do see the exact behaviour you describe.
But setting Photoshop's greyscale space to a proper ICC profile, like SWOP v2, so that it uses the black ink behaviour of that profile, I get something more like I would expect. Using Sheetfed Coated v2 as source, and using Rel Col, without Black Point compensation to convert to the Black ink bahaviour of SWOP v2, both 100K and a 75C 68M 67Y 90K map to 100K. This makes sense to me - without BPC no L* scaling is taking place, so the darker rich black clips to the darkest value available, 100K. Doing the same conversion *with* BPC, the tonal difference is maintained, with 100K converting to 97K, and the darker rich black converting to 100K.
So if one have a CMYK document where you want 100K to convert to 100K greyscale, it seems the way to achieve it is use a proper ICC profile for the destination greyscale profile and don't use BPC. But of course this may lead to loss of shadow detail.
Matthew Larmour
> A C0, M0, Y0, K100 patch in CMYK -- say using U.S. Sheetfed
> Coated v2 -- is *not* the darkest black you can get within that
> color space. That is why, upon conversion to Grayscale, it does
> not translate to K100: because there exist possibly darker
> tonalities than that in the CMYK space.
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