Oh boy, you’ve likely seen this once in your career, and with luck it on was on color proof and not at the press check. Black on Black. 100% Black on Rich or Super Black (Black with CMY added to darken it more). Most cases of these coming up is because you weren’t paying attention to the black background of a photo edited photoshop and you forgot to match the black in Indesign or Illustrator to it.
Photoshop by default has this wild idea of what the blackest of black should be: C75, M68, Y67, K90. Now how Adobe decided that’s the CMYK equivalent of R0 G0 B0 is beyond me, I can’t imagine any printer printing that or any paper thinner than cardboard holding that ink without curling. Most printers use C60 M40 Y40 K100 for a Rich Black for coated cover stock, and you can set Photoshop to always aim for that number when setting black points.
With NO documents open, hit ‘x’ so your Foreground, Background swatches are black and white and double click on the black. Set your numbers to C60 M40 M40 K100 and click OK.
Now create a new document, and go to Image > Adjustments > Curves (or Levels) and double-click on Set Black Point. Color Picker will come up and again input the Rich Black settings. Click OK, and OK again at which point Photoshop will ask if you want to save this as the default targets, Click Yes. Close Photoshop to save all these settings. Now when you choose Black for a fill you will get your C60 M40 M40 K100, when ever you chose a shadow area as the black point, that pixel will be pushed to the same.
If you prefer to not use Rich Black, do the same steps removing any CMY from the black point values. You can achieve similar results by customizing your UCR/GCR settings under color management but I would advise against it, as it will alter your ICC profiles.