The answer is indeed debatable... I have an early 68 convertible built Jan 6 1968 at Metuchen, NJ. When I bought the car in 2004, it had the original carpeting. It was dark blue. When I went to replace the carpeting, there was no dark blue offering at any of the major Mustang parts places, just the medium "Ford" blue. Puzzled, I contacted the MCA head judge via email and asked him the question. He replied that dark blue very well could have been the original carpet color and Ford was on strike when my vehcile was built, and they could have used leftover 1967 dark blue carpet (among other possible leftover 67 parts) just to complete the build.
Most 68's with 2B interior have the medium "Ford" blue carpet, but some indeed exist with factory installed dark blue leftover 67 carpet.