I went with a Flow-Pro 765 (FP765 Center and Offset - these mufflers are 100% reversible too, mind you) on mine. Make sure you stick to the 2.25/2.5 " pipes and you should be okay. The heads already flow very well from the factory - the marginal increase you would get from changing the headers out would not warrant the cost IMO. As for the Flowmasters? They sound good to a point, but get a little drone-like and buzzy on the 4.3's. The only thing that sets Flowmaster apart from the crowd of "bolt on performace" is that they have a better, well paid marketing division.

The Flow-Pro on mine makes it sound like a slightly hopped-up Yukon with the V-8. Good tone, not so loud as to grab any unneeded attention from the cops - and improved my gas mileage by about 11% on average since the day it went on from the stock muffler. I should take some sound clips one day... break out the studio mic and the portable digital recorder...
There is also good opinion and experience with Ravin's offering in mufflers too. Ask or wait for BlazerLT to come around to clue you in on that.
EDIT: For final thoughts and Typo in the part number for Flow Pro (Sheesh)
Fill us in on what year and model of Blazer you have. To reiterate for a person that posted below - CPI fired models made in '95 DO NOT benefit one jot from headers. In fact, can cause some harm with carbon fouling issues and totally throwing engine temperatures way off. You end up messing terribly with the flow in the heads of my particular truck and you end up doing engine damage to include burned valves and seats - you name it. Not every solution works in *every* situation.