You are correct in your supposition about those "black" officers. Most of them could have passed as white in any place where they were not personally known. In most cases, you would not recognize a quadroon as a black person, and an octoroon would be considered white anywhere that their family tree was not a matter of public knowledge. Here is a relevant quote from a Yankee naval officer who passed through a contraband camp during the War in Louisiana: "I visited the different Negro camps [below Fort DeRussy] today in company with 3 of our officers and found nothing but what would disgust me with the whole race of blacks unless it was one beautiful octoroon girl who was among them but considered herself no better than the rest of them . . ." A Union soldier passing through Opelousas during the War was very concerned about large numbers of army-aged men he saw loitering about the town, but was told that they were not in the army because they were black. He commented to the affect that "if these men were black in the eyes of the law, then the eyes of the law were much sharper than ours were."