After Morrow's second goal of the game forged a 2-2 tie 2 1/2 minutes into the final frame, Richards tipped in Trevor Daley's shot 4 minutes later to put the Stars ahead for good.
The Blues, who entered the contest on a five-game winning streak, took the loss despite first-period goals from David Backes and Alex Steen.
Jaroslav Halak allowed all four Dallas scores on 27 shots.
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