1997 blazer, sparks but won't start just backfires.

Have you check to see if the timing on the car is right?

Had an 84 silverado 305 that did the same thing. Try giving it a full tune up. Plugs, wires... Mine turned out to be fouled plugs. I didn't find that out till after i tore it down & put a timing set in it. Wasn't getting good enough spark.

It needs at least 60psi fuel pressue to start. You need to check fuel pressure with a gauge.
If the dist cap or rotor is bad, the spark will not be on the right plug at the right time. Same goes for a faulty distributor or a problem with the timing chain. And the ignition module.
Also check the ecm for trouble codes.

before u replace the whole dist. have u first checked for fuel delivery on the carb or fuel rail, and then if u have that then check the spark plug wire by removing one while cranking by arcing to metal , if you see spark then dist is good if not then replace, but if dist is good and still no start then check coil pack and or all fuses

Check the plug wire from the ignition coil to the distributor cap.
If you haven't already, replace the distributor cap and rotor.
The GM Vortec's have issues with moisture getting inside the cap and rotor and causing problems.

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I think you have a valve problem. Need to remove valve covers and look for loose or broken rocker arms. If an exhaust valve does not open because the rocker is broken the trapped combustion gasses will revert through the intake when the intake valve opens. This can interupt the intake flow to other cylinders and prevent starting.

if it has a distributor, it is most likely the ignition module inside thats shot. call an auto parts store. and I think they can test them to be sure if its bad or not. check the cap and rotor also ! you can turn dist. a bit if timing is off. theres a hold down bolt underneath dist just loosen it. just so you can turn it to where it runs the best! then tighten and try it. if not the module. make sure power is going to dist. also make sure dist. shaft is turning, in dist. hope this helps.

try spraying wet start on the cap leads and coil.

Check for injector pulse if no pulse with no spark check ignition module in distributor or crank sensor.

There may be no injector pulse if the computor does not see a RPM signal.
There will be no spark if the ignition module does not see a dist pulse from the dist pickup.......a RPM signal.
Because you do not have both ......I would suggest to check/change the dist pick up.
Follow the wires from the ignition module inside the dist to the pickup which is also inside the dist.
If it is just a bad dist pickup....keep all your old parts as good spares.
Also...is the dist turning? to provide a RPM signal?
You will have to remove the dist to change the pick-up.....mark your timing locations......"rotor position" and "dist body location"before removal.
A quick test before installing the "repaired dist" .....plug it in the the truck wiring and turn on the key....DO NOT CRANK THE ENGINE OR YOU WILL LOSE YOUR DIST ROTOR CORRECT POSITION......as you spin the dist by hand .....the injectors should spray.