If your artwork has a lot of heavy deep blue areas chances are real good that it will look like purple when it is actually printed. When deep blues are built from process colors they are built primarily out of cyan and magenta.
Since most printers gain very heavily in the magenta, when your job is printed it will often turn from blue to purple. If you have the ability to do so, consider cutting the magenta in your blue back 15% or mention doing so to your designer when sending to your printer. This will insure that your blues actually look like blues when they are printed.