Going into the game Southend had the worst away record in the division but they took full advantage of a Blues side that lacked any finishing power.
Southend, who had only scored five goals in their previous 14 away games, took the points with goals from skipper Kevin Maher, Eastwood and Mark Gower.
The Shrimpers, beaten 4-0 at home earlier this season by Blues, could have found themselves in a similar situation in the opening 20 minutes but they rode their luck to storm their way to their unexpected success.
Blues, playing their first league game since New Year's Day, were often far too anxious but it was still a frustrating performance as they lost further ground on leaders Derby County.
Early in the game it appeared as if Blues were in complete control when Martin Taylor used his height to produce a powerful header from a Seb Larsson corner that was brilliantly saved by Darryl Flahavan.
But Flahavan was completely wrong-footed in the eighth minute when he went for a cross form Larsson only to be left stranded as Peter Clarke chested the ball agonisingly into an empty net.
Larsson continued to be a thorn in side of Southend. The on-loan winger smashed the ball against the post with Flahavan completely beaten.
On his debut newcomer Rowan Vine demonstrated an eagerness to get on the score-sheet with a shoot on sight policy which kept the Southend defence very much on their toes.
Southend's 22nd minute equaliser was a comedy of errors with goalkeeper Maik Taylor taking the blame. He advanced from his goal to clear only to smash the ball into the face of Mat Sadler.
Jamal Campbell-Ryce gained possession and set up a pass for skipper Maher to capitalise on the situation with a low shot past the embarrassed Maik Taylor.
Southend certainly had little trouble adapting to Blues new pitch as they were boosted by their out of the blue equaliser.
Eastwood was not such a threat as expected in the first half but the Southend striker is never more dangerous than when seemingly out of the game. He struck in devastating fashion in the 47th minute when he collected a headed pass from Lee Bradbury turned inside and slotted home a positive shot.
Blues strived for the equaliser and Bruce sent on Stephen Clemence and Fabrice Muamba for the struggling duo of Bruno N'Gotty and Mehdi Nafti.
Flahavan distinguished himself with a flying save to keep out a Vine header and this proved to be Blues' final effort as Southend completed their humiliation when Gower notched their third goal with a blistering cross-shot seven minutes from time.