The blue used on the 66-67 Shelby Paxton blower unit is Ford Corporate Blue, same color used on the engine itself. If you are spraying this color out of an aerosol can, there are many different brands and color variations. Plasti-Kote #224 seems to be a good match, but there are many others.
If you are using a professional spray gun, which gives the best results, you will want to use:
PPG Delstar DAR 13358
Do not use any flattening agents like you would when applying to an engine block. The color should be Glossy Ford Corporate Blue.