Bruce - who led the Blues to the Premier League twice during his spell in the dugout - saw Sone Aluko return to haunt Lee Clark's side with a brace, as the Tigers raced into a 3-0 first-half lead.
The Blues hit back with two goals either side of the break but Hull held on to complete only their second win at St Andrew's in over 100 years
Bruce said: "It was an unbelievable turnaround. But credit to Birmingham they stuck at their task just when I thought the crowd would turn against them.
"The team talk at half time was to keep it tight and they scored again after 12 seconds. It just shows what rubbish you talk.
"But I thought some of our football in the first half was very good. I have some really good players and the way Aluko is playing, is there a better player in the championship? He has been excellent for us."