This is one of the most poorly understood print production issues and it can have significantly detrimental impact upon your printed piece. Thankfully it’s actually quite simple and easy to resolve!
Each paper has different absorbency level effected by both the paper stock and coating. In general, most paper can support a combined total (when you add up your percentage of C M Y K) of around 300%. Poor or uncoated paper tend to support less (try 270%) but high quality paper used in fine art prints can be more (some up to 330% or higher). 300% is generally a safe number but you should check with your printer if maximum color intensity is important.
When printing rich blacks, you should generally have 100k and split the other 0-200% between C M Y as desired. A good general setting is 60c 50m 50y 100k (more C then Y & M as the black tends to look a bit odd with more M or Y). Registration black should not be used as this provides 100% for all 4 color channels totaling 400%.
What will happen if I use to high of an ink density?
If you exceed the mediums maximum output in ink one of two things will happen. 1. Too much ink will be placed on the paper and you’ll have ink spread (not pretty). 2. Their Rip or pressman will catch it and adjust the images to be within tolerance. You have no idea how they’ll adjust it and you never know what you’ll get. A great tip to adjust a 4 color image with blacks that is over 300% is in Adobe Photoshop, convert from CMYK to LAB and back to CMYK. You’ll notice that no values under 300% have been changed at all and all blacks retain 100K as well as properly reduced C M Y. Simple yet effective.
Minimum ink percentages per channel
Ink percentages should not be to low either. Generally you should not have a percentage value of less then 3% or any given channel (C M Y K) as the press will likely not print it at all if it is but you can check with your printer for their equipment specific tolerances.
Newspaper ink percentages
Greyscale Newspaper generally have even tighter tolerances due to the poor paper quality. Safe values tend to be between 7%-90%K but there is much variation in this so you should check with the publication. If you have subtle differences in grey such as something 85k next to 100k this will be a major problem if the rip changes it from 100k to 90k. What was a 15% contrast just became 5%, nearly the same so best to make the adjustment can keep proportional differences. Deal with the fact that what you want black will only be dark grey as this is a reality of newsprint and adjust your files accordingly.