My mate has a 2004 Blazer which starts and runs fine for an hour or so, then the engine stops and won't start. After another hour or so it is fine again.
He has consulted Chevy dealers and "experts" and changed sensors and the like but nobody has solved the problem, going on a year now!.
It looks like something electrical is failing when it gets hot, then returns to normal when it cools off again.
One person told him there is a known fault with a resistor behind the fuse box which tends to fails in this way, but we haven't been able to locate such a resistor, if it exists.
The OBDI readout doesn't show anything helpful.
Can anybody please help, he is at his wits end, and I am not familiar with the Blazer, I have a Spark.

 

Maybe try changing the thermostat??? Just a thought on my end... "shot in the dark I guess"

 

The temperature gage shows normal all the time so I don't think it is overheating.

 

I am racking my brain trying too figure out why it seems too be an electrical problem once the truck has ran for an hour or so??? One would think that it would happen well before that time range? I have no clue honestly but someone with much more knowledge will surely help you out soon

 

As you have spark check your fuel pressure per fuel pump test described in the second gen stickies when it won't start, sounds like the fuel pump is getting weak and maybe over heating and shutting down. Been there done that. I've had several in my shop over the years.
Edit;
If you find the fuel pump is not able to run before you go thu the headache of dropping the fuel tank check for power to it and the relay again,
Swap relay out with a known good one.
At first switch of the key on position make sure you have power to both hot spots in the relay terminal mount. { at first key on the pump only has power ran to it for a few seconds to pressure up the fuel system }
Make sure there is power all the way back to the pump.
I don't know if a 2004 has it but there should be a pump test wire on the fire wall up by the brake booster. if so put power to it via a jumper wire and see if pump runs. You can also test by jumping the pump wire in the pump relay terminal mount.

 

If it's loosing spark, could be a bad ignition control module (little silver thing mounted next to the ignition coil on the passenger side of the engine on top). They're susceptible to heat, hence the aluminum fins on them to help keep them a little cooler, and I've seen them fail with the characteristics you describe.

If it's loosing fuel, it could be a failing pump that's overheating, OR possible vapor lock in the tank (happens when the evap vents get clogged for any number of reasons. as the engine runs and fuel is pulled from the tank, no air gets into the tank to replace the space, so a vacuum builds to the point where the pump cannot overcome it and keep proper fuel pressure). Next time it does it, jump out and pop the gas cap. If you get a "woosh" of air sucking into the tank when you pop the cap, try to start it then and see what happens.

 

I fought this same thing for 2 years on my jimmy. I was convinced it was the ignition module. It would only do it when the ambient temperature was above 90. Last spring my fuel pump died. Since replacing the pump it ran perfectly all summer no matter how hot it was outside.