Midfielder Black was red-carded by referee David Somers after picking up a second booking in the 86th minute after a tackle on Gavin Morrison.
"The sending-off was extremely harsh," said McCoist, whose side picked up the three points at Borough Briggs thanks to Lee McCulloch's 73rd-minute penalty.
"He got booked for three or four fouls, the ref said, and rightly or wrongly he was on the yellow card.
"We told him he's got to stay on the park and stay on his feet, and I thought he did.
"The game was hard-fought but I don't think there was a bad tackle in the whole game. It's just because it's changing, the laws of the game are changing to the detriment of the game.
"We've got a player sent off today, in my opinion wrongly. I thought the ref handled the game very well but sadly I think he got that one decision wrong.
"I was tempted to sub Ian because we need 11 men on the park but as it happens we got away with the sending-off. I felt there was always a big possibility of him getting sent off after receiving the yellow card, just because of the way he plays the game.
"But to counter that I did think it was extremely harsh."