Kyle Bartley scored in the 94th minute to cancel out Daniel Ayala's strike that looked to have given Boro a win at St Andrew's.
Clark told the club's official website: "When you leave it so late, you're happy that you get the positive result. It's another point on the board and five games unbeaten.
"Kyle was desperate to get on the field and he's come up trumps again, he's scored some important goals for us."
Birmingham are 19th in the Championship following their draw.