RGB blue is a vibrant and beautiful blue--however--because we cannot print in RGB, the color has to be converted to CMYK. When RGB blue is converted to CMYK, it becomes a combination of 99.6% Cyan and 95.7% Magenta. This conversion results in a ratio of Cyan to Magenta that produces a purple hue, and this is how it will print.
RGB blue is outside of CMYK's gamut: CMYK simply cannot reproduce that shade of blue. The computer and software substitutes the closest color to it. Technically, that color is purple. It's best to choose your blue in CMYK, and to be careful how much Magenta is in the values range––especially if the Magenta is as high as, or close to the same value of Cyan.
For covers of Black & White titles, we can accept an RGB file, and will convert it to CMYK before printing. LSI assumes that the designer of the file is aware of this RGB to CMYK conversion, and the potential to affect all colors (not just blue). We don't assume responsibility or make any adjustments to help a color maintain its original appearance. Most colors convert reasonably well from RGB to CMYK. These colors are within the overlapping gamuts of each color space. Unfortunately, there are many colors that do not convert well. If the publisher/designer is concerned about how a file will convert, they should perform the conversion and make necessary adjustments before submitting files to Lightning Source.