Little Contrast in Dark Tones

Black is printed using a lot of ink so there is very little nuance between very dark tones and grays. When you see your design on the screen these differences may be visible, but they can easily turn into a big black nothing during printing. Best case scenario, it will look like a black cat on a black sofa, worst case like the Cheshire cat in Alice in Wonderland, where the image slowly disappears and you will be left with nothing but two glowing eyes on a black background.

Better to have a lot of contrast than too little.

See "Little Contrast in Saturated ColorsLittle Contrast in Saturated Colors"