2002 chevy Blazer hard starting did change fuel pressure regulator and fuel filter when we spray starting fluid in throttle body it will start.

This sounds like the fuel pump is shot. The pump has a 'dead' spot and when the engine is started by using 'nasty' starting fluid it will go to the running mode and run fine. Never use starting fluid!!! A bit of gas is best, just use common sence. Frankster016

Replace the fuel injectors, that is the likely cause......

to test the injector, take the power clip off the top of the injector, take one wire and ground it to the body of the car, and another form the battery, to the other post on the injector, if you do not hear a pulsing sound coming from the injector it is bad, if you do, then you have a wiring problem.

I agree with the fuel pump problem this is quite common with these trucks. but the fuel injectors on this motor are not accesable without removing the plenun and they are not standard fuel rail injectors so you can not test as earlier instructed.

Have a helper turn the key on while you listen for the fuel pump at filler tube. check fuel pressure, should be 55-60psi. you can get a cheap fuel gauge at autozone etc for about $20 . the connector for testing is on the fuel rail close to the lines that feed into the intake. if you don't have enough pressure, or no pressure, try swapping relay with identical relay in the under hood fuse/relay box. Listen for it to click when the key is turned on. Good luck.