Dopamine blockade isn't significantly induced by doses of Seroquel used for sleep. Although it and its metabolites do have action at certain dopamine receptors, lower doses will first occupy other receptors that they have a higher binding affinity for. Its metabolites are partial agonists at 5HT1A autoreceptors that induce downstream release of dopamine, which theoretically reduces the risk of tardive dyskinesia from direct dopamine blockade.

If you're curious about this, Stahl's Essential Psychopharmacology textbook goes into detail about this and is written in a way that some idiot (me) on HN can grok it.