Avoid that purplish cast in your blues with the right CMYK mix for printing.
Dark blues turning into a slight purple has been the bane of graphic designers everywhere since William Kurtz patented the first use of three separate printing plates (Cyan, Magenta, Yellow) in 1893. (See my infocard Tints and CMYK).
Computer monitors also make the color selection complex since it uses RGB instead of CMYK to display colors. (See my infocard Computer vs. Printed color).
The best rule of thumb is to have at least 30% difference between Cyan and Magenta.
At Cyan 100% (C 100) and Magenta at 70% (M 70) or less, your blue will not print with a purplish cast. If Magenta gets into the 80% or 90% ink coverage, the blue will print with a slight hue of purple. If you need a darker blue, add a percentage of Black (K). A 10%-50% black ink coverage will turn that medium blue into a rich, dark blue when printed.
Programs like Adobe Illustrator (vector) and Photoshop (images) make it easy to change CMYK percentages and view the difference on-screen. (See my comparison of vector graphics and images PDF).