Does buspar (buspirone) affect serotonin by blocking it?

No: Buspar has several effects. One is a weak blocker of dopamine. The main effect is probably as a serotonin 1a partial agonist which is a different type of serotonin receptor than ssri antidepressants, that has multiple effects including calming without sedating. This is a mechanism that is shared by Abilify and viibryd (vilazodone hydrochloride). It has been shown to be helpful as an add-on to ssri antidepressants.