Officially, it's Ford Corporate blue.
Most have noticed a difference in the shade over the years though.
The official EXPLANATION is that it is a "batch" color. What Ford means by that is that there is no official formula for it and the shade varies from batch to batch like some of the primers do as can be seen on the underside of the floor pans on production models.
VHT paints long ago had two shades of it. Ford blue and "Mustang blue". The Mustang blue is what is more commonly seen by me on 302's and 351c's and from VHT is the darker of the two colors.
It can and does vary though depending on the engine plant assembling the engine. Cleveland v. Windsor for example.
I personally prefer the "Mustang blue" but that is just personal preference. MCA doesn't have a points penalty for using one shade of blue vs. another simply because of the normal discrepancies possible directly from Ford to begin with.