All of the original 1968 1/2 Mustang hoods that I have seen have been semi-gloss black.
The brittany blue hood in the attached picture is the origal from my late April Dearborn built car with (then) 26,500 miles. The two green hoods are from original cars and the attached snip from a feature article in the November, 1968 issue of Rodder & Super Stock magazine clearly shows semi-gloss even though they called it flat black.