Foster cost Birmingham £6 million from Manchester United in the summer and yesterday went a long way in paying some of that fee back.
Craig Gardner hit a brace for the home side after Steven Nzonzi's opened the scoring.
Sam said: "Ben Foster won them the game with his outstanding saves and our inability to convert chances into goals. We've completely dominated the game, created far more chances than Birmingham but failed to take them. And to boot we go and throw it all away.
"It's a typical Achilles heel of ours, not being able to take advantage of dominating the game and creating chances to win it, and we've paid the price."