Henning Berg's side started brightly and took the lead from the penalty spot after 13 minutes.
Blackburn looked dominant in possession but were quickly pegged back by an incisive Birmingham breakaway. Bright work out wide from Chris Burke found Nikola Zigic unmarked in the box. His header hit the post but King was on hand to poke home the rebound for his fifth goal in as many games.
Danny Murphy hit the side netting with a strike from range as the half drew to a frustrating close for the home side.