photo black

I haven't used the photo black in some time, so I decided to make that switch from matte to photo black. This resulted in a big black ink mark mess on one test paper after another with the nozzle check showing many gaps on each channel. I switched back to matte black, but still have ink marks...

If you print photos (on photo paper) then you use the dye-based black (denoted K often times I gathered) instead of the pigment-based black (Bk). Now when does this "photo black" actually get used? Is it only for the parts of the photo where it is completely black ("true black")? Or does it also...