Short answer
Because one layer of black ink is not very black, and the paper decides how black any ink can be. A solid made from black ink alone, an uncoated or matt stock, a desktop printer holding ink back, or two different black builds side by side each give a black that reads as charcoal. On coated paper the answer is a rich black inside the printer's ink limit; on uncoated paper the black reflects several times as much light whatever the file says, so the answer is the stock or the expectation.
1. A large black area is built from black ink alone
Test: Read the values of the black in the file. If it is 0/0/0/100, compare the solid with the darkest shadows of a photo on the same sheet; those use four inks and will look deeper.
One layer of black ink does not absorb all the light. It is right for text, but over a large area it reads as a dark grey beside any four-colour dark.
2. The paper is uncoated, matt or absorbent
Test: Lay the print beside the black on a glossy magazine cover or photo print. If that black is clearly deeper, the paper is setting the limit.
Ink sinks into uncoated fibres and the rough surface scatters light back, so the darkest black an uncoated sheet can carry is several times lighter than on coated stock.
3. Two different blacks are sitting side by side
Test: Look for a rectangle where a placed photo or logo meets the black background. A visible edge means one black is a four-colour mix and the other is black ink only.
A black converted from RGB becomes a mix of all four inks, while a black typed into the layout is usually black ink alone. They look the same on screen and different on paper.
4. A desktop printer is set to plain paper, draft or economy mode
Test: Reprint a small black patch with the paper type set to match the sheet in the tray and the quality set to its highest. If the black deepens, the settings were holding ink back.
Printer drivers limit ink for plain paper and draft modes so that sheets do not cockle or smear, and the first thing lost is the density of black.
5. Light is reflecting off the surface
Test: Tilt the print so that no lamp or window reflects in it, and shade it with a hand. If the black deepens, surface reflection was lifting it.
Every surface reflects a little of the light that falls on it before the ink can absorb it. On a black that small addition is most of what reaches the eye.
Causes are ordered by how often Colourwise expects each to be the explanation — an editorial judgement, so test them all.
Grey black is the opposite fault from brown black. Brown, blotchy or smeared black comes from too much ink and is covered with muddy prints; grey black comes from too little ink, or from paper that cannot hold it. Start by looking for a deeper black on the same sheet. If photographs have rich shadows while a flat panel beside them is charcoal, the panel is black ink alone and the fix is in the file. If nothing on the sheet is darker than charcoal, the ceiling is the paper or the printer's settings. And if the grey appears as a rectangle around a placed image or logo, there are two different blacks in the layout, which is a third problem with its own cure.
The table on this page takes the black solid from four published printing conditions. On premium coated paper it is L* 16, which means it still reflects about 2% of the light that falls on it. On wood-free uncoated paper it is about L* 33, reflecting about 7% — three and a half times as much — and on newsprint about L* 37 and 9%. Those are the targets a well-run press aims at, not faults. A design that depends on a deep black therefore needs a coated sheet, or a gloss laminate or varnish over the print. On uncoated stock it is better to treat black as the soft, dense charcoal it will be and choose the other colours to suit.
A black box around a picture is the most recognisable version of this problem. A black that arrives in an image converted from RGB is a mix of all four inks. A black drawn in the layout program is normally black ink only. On screen both are the same pure black; on paper the four-colour one is visibly deeper and often warmer or cooler, so the edge of the image shows. The cure is to make them the same build. For large areas that means one rich black used everywhere, including behind images, with its values taken from the printer; for text and fine rules it means black ink alone, which stays sharp because nothing has to line up with it.
An office or home printer generally decides how much ink or toner to lay down from the paper type and quality chosen in its driver. Draft and economy settings exist to use less, plain-paper settings are cautious about how much ink ordinary paper can take, and the density of black suffers first. Before blaming the file, set the media type to match the sheet that is loaded and the quality to its best, and reprint a small patch. Paper counts here as much as it does on a press: expect a deeper black on a coated inkjet or photo paper than on copier paper, for the same reason a magazine cover is blacker than a newspaper.
| Printing condition | Black solid L* | Light reflected | Against coated black | CMY overprint L* |
|---|---|---|---|---|
| FOGRA51 — Offset, premium coated paper (PSO Coated v3) | 16 | 2.1% | 1× | 23.26 |
| FOGRA52 — Offset, wood-free uncoated paper (PSO Uncoated v3) | 32.69 | 7.4% | 3.5× | 34.61 |
| FOGRA39 — Offset, gloss or matt coated paper (ISO Coated v2) | 16 | 2.1% | 1× | 23 |
| IFRA26 — Coldset offset on standard newsprint | 36.76 | 9.4% | 4.5× | 40.39 |
Why: The background is black ink alone and the photo shadows are four-colour.
Fix: Rebuild the background as the printer's rich black and keep it the same behind every image.
Why: Uncoated board absorbs ink and scatters light, so its darkest black is several times lighter than coated.
Fix: Accept it as the character of the stock, or change to a coated or laminated board.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
The published black solid is L* 16.0 on premium coated offset paper (FOGRA51), L* 32.69 on wood-free uncoated paper (FOGRA52) and L* 36.76 on standard newsprint (IFRA26).
Source: FOGRA51 characterization data (premium coated offset, ISO 12647-2:2013); FOGRA52 characterization data (wood-free uncoated offset, ISO 12647-2:2013); IFRA26 characterization data (coldset offset on newsprint)
Colourwise analysisStrong evidence
Converted to luminance factor, those solids reflect about 2.1%, 7.4% and 9.4% of incident light, so the uncoated black returns about 3.5 times and the newsprint black about 4.5 times as much light as the coated one.
Based on: The standard CIE relation between L* and luminance factor Y applied to each published black solid; see the table.
Source: FOGRA51 characterization data (premium coated offset, ISO 12647-2:2013); FOGRA52 characterization data (wood-free uncoated offset, ISO 12647-2:2013); IFRA26 characterization data (coldset offset on newsprint)
FactStrong evidence
In the FOGRA51 data, all four inks at 100% together are L* 12.7, darker than black ink alone at L* 16.0, while ISO 12647-2:2013 keeps the tone value sum on coated paper below 330% and never above 350% for sheet-fed offset.
Caveat: A usable rich black sits between the two: darker than black alone, well inside the ink limit.
Source: FOGRA51 characterization data (premium coated offset, ISO 12647-2:2013); ISO 12647-2:2013 Graphic technology — Process control … — Part 2: Offset lithographic processes
Colourwise interpretationLimited evidence
A visible box around an image on a black background is nearly always two different black builds meeting, not a fault in the image or the press.
Based on: Colourwise reading of the difference between a four-colour black and black ink alone in the characterisation data, and of how RGB blacks are converted compared with blacks specified in a layout.
Caveat: Not a measured frequency; overprint and transparency settings can also draw an edge.
Reviewed 1 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.