The presidential blue paint code that would have appeared on a 1968 Mustang driver's side door data plate would be "X". The ditzler paint code number is 13356. This should be enough info for the paint shop to get you the proper paint color. Are they planning to use PPG paint?
Here's some good info on this site:
1968 Mustang Paint Color Codes - MustangAttitude.com Data Explorer
I'm going through this right now with my '68 coupe which is going to be painted highland green..aka bullitt green.
Good luck...presidential blue is a nice color!