The history of the PK ink is a bit convoluted.
Originally Selenium 1 was the default PK ink and then IJM developed a new ink (Warm Neutral 1) which was a 100% carbon "opaque black" ink for dig-negs (original "PZDN" system). Quickly it became evident that WN1 was a better gloss/dmax than Selenium 1 and the switch was made for WN1 to become the default.
Similar things have happened on the Matte Black side and added to this complexity is our new HD black inks (UltraHD-MK and HD-PK) that we now offer master Piezography curves for as a drop-in replacement to the old ones.
That's the recent history.
Long story short we offer 4 black inks for Piezography
a. Piezography Photo Black (originally known as WN1 but changed to just Piezography Photo Black in late 2015) which says "Opaque Black" in small letters on the label.
b. Piezography Matte Black (originally known as Neutral Shade 1 but changed the name to Piezography Matte Black in late 2015)
The new ones (don't worry the originals will be available for a long long time if not forever):
c. Piezography UltraHD-MK (this is a freakishly dark 100% carbon matte black that is part of the pro inkset but compatible with K7 that gets to dMax of 1.85 on some papers. This can be a drop-in replacement to the original MK and you can just re-linearize whichever curve you primarily use for printing. It's incredibly expensive to make and we are working on batch #2.)
d. Piezography HD-PK (This is also an incredibly dark Photo Black and it requires a new master curve that correctly compensates for how extra glossy this black is compared to the other "legacy" K7 piezography shades in the printer.)