Hi,
I have a 2000 Blazer with the 4.3 vortec engine. When I accelerate the serpentine belt sequels. When looking at it idling I notice that the idler pully(I think thats what it is) moves back and forth is this supposed to do this or do I need to tighten it?

The tensioner is a spring loaded pulley that acounts for slack in the belt system. There are no adjustments.
it might be time for a new belt. Also check the engine for leaks that might be getting on the belt.