I looked in the easiest place to look: Heitman's Historical Register of Officers of the Continental Army. Several Blackman's but all from Connecticut. But not finding your man there just says Heitman didn't list him. Heitman listed officers, BUT only of the Continental Army. He didn't list Militia or State troops. I don't know the south well, but my impression is Heitman doesn't do a good job with Continentals in the south.
Because of my northern focus, I can't point you to a good book that lists South Carolinians in the revolution, but I'm sure they exist. Any good genealology library ought to be able to put you on the track of such a book.
Marion was both a Continental Officer and later a Brigadier General of State Troops. It's probably the later period in which your man served. He may not have served as a Colonel under Marion, he may have ended his life as a Colonel (or Lieutentant Colonel) and the story was passed with this highest rank. That happens a lot.
To Join the Company of Military Historians click here