I do not think it was Shelby Blue because that was either on his 1965 cars Guardsman Blue or according to my friend John Allen Caspian Blue. But FAV was formed before Shelby took over racing in 1965 and the Big Blocks in 1965 could be a Shelby colour but the 1964 prototype was a dark Blue with black sills and looks different to the 1965 Big Blocks.
Claude Nathaune (Sorry about the spelling) made a receation of GT/101 the 1964 prototype I saw it at Goodwood and it locked Matt Black . When I asked him the coulour he assured me it was a Matt Dark Blue.
I have read the great "GT40 anthology" book by John Allen and Graham Endeacott. John Allen says that the seal stripes on G/101 were dark blue.(not Black).This might have something to do with the colour Photography at the time.