Sometimes when a client sends/brings me a file they think they have a blue color because it appears blue on their monitor, but I know it may print purple. When using blue in your designs, always make sure to leave at least a 30% difference in your Cyan and Magenta values. For example use, C-100 M-70 Y-0 K-0; this allows enough color differentiation to ensure that your blues don't look purple when printed.  If you can't do this in your graphics program, just ask us to check it for you.