Rich black vs 100% K black
Flat K-only black prints as washed-out grey over large areas. Rich black adds other inks underneath so it reads as truly deep black.
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What it is
100% K is one ink: the black plate at full strength and nothing underneath it. Perfect for text. Spread that same value across a full-page background and it prints as soft charcoal, because a single layer of ink can only get so dark on paper. Rich black stacks the other process inks under the K so the area drinks more pigment. A common build is C60 M40 Y40 K100.
Use 100% K for anything small: body copy, hairline rules, barcodes, captions. Four plates that shift by a hair leave colored fringes on every letter, and small type is where that shows first. Use rich black for big solid fills, covers, packaging panels, and any black sitting next to a photo whose own blacks are already built from four inks.
Two traps. TAC (total area coverage) is the ink cap your press can take before the sheet stops drying and starts smearing, usually 280 to 320%, so ask the shop for their number instead of piling on 400%. And 'registration black' (100% of all four inks) is a prepress marker color used for crop marks. It is not a design color. Never fill artwork with it.
Ask AI for it
Set up the blacks in this print file two ways. For all text under 18pt, thin rules, and small marks, use 100% K only (C0 M0 Y0 K100). For large solid areas, covers, and background panels, use a rich black build of C60 M40 Y40 K100 and keep total ink coverage under the printer's published limit (usually somewhere from 280% to 320%, so ask for their number rather than assuming 300%). Do not use registration black (C100 M100 Y100 K100) anywhere in the artwork. Work in CMYK, not RGB, and flag any pure-black elements that sit on top of a rich black field so they can be knocked out rather than overprinted.