Your Marti shows Deluxe Belts which were color keyed to the interior. The Dark Blue are the correct ones for your car. My last 68 was a Light Blue Decor Interior and it had Dark Blue Deluxe Seat Belts. Black was the Standard Belt for All Interior Colors unless Deluxe was ordered.
Of Course, you never know what Ford might have done, but since the Black Belts are Dated later then your Car Build Date then they were added sometime later. Andrew