H6C = H high trim grade (Pxx would be Premium), 6= bucket seat (5=bench), C= interior color, in this case it was two tone - blue and white (see below). The "U B S" to the right of the trim code U is (Upper) door frame color. The others B and S are normally blank in a 67 B-cuda. I'd guess since you said dark blue it is an U-"E".
