On December 15th of 67, GM issued a zone letter indicating four colors were being dropped from production and four new colors were being added effective January 2nd 1968. The blue that was discontinued was "Fathom Blue" as they had just introduced "Lemans Blue" into the mix. They did not discontinue Grotto Blue and it was still in production until the end of the production year if I stand correct?. Here is a copy of the letter Jeff C. posted showing the color changes in early 68.
This should clear up the Blue issue in the Grotto.