1996 chevy blazer will turn over but not fire. I have changed cap rotar plugs wires and fuel filter.what next

yes

No spark at the coil replace all spark plug wire and coil wire but start with fuel pump first it wont start anyways without fuel id look into a new cap and rotar with the plug wires might get lucky and find a distributor at a wreckers for the same price as cap and rotar

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.

I’m happy to help further over the phone at https://www.6ya.com/expert/jason_00b31cc8f593773a

change the coil packs

First spray starting fluid in the air filter. If it starts it is a fuel problem. If it doesn't start then pull one of the spark plugs and see if there is spark. If there is not spark then it is an ignition problem.
If it is a fuel problem and the fuel pump is running when you turn the key on, then check fuel pressure. It should be over 35psi.
If it an ignition problem start by checking fire at coil.

Coil, ignition fuse, or a bad rotor/cap issue.

All GM have ignition coil problems , your getting too much fuel , I have seen O2 sensors and plugged catalytic mufflers too many times , be careful the next ride the vehicle will catch on fire..Try the coil first

Barring any mechanical issues you most likely have a failed injector.

Had the same problem with by 1998 S10 a couple years ago. Changed fuel filter?? didn't work. Changed the fuel pump?? didn't work. Changed the fuel pressure regulator?? didn't work.

I was about to the end of my rope when I put a timing light on it and it showed firing on cylinders more then once a cycle. Turns out my distributor cap was bad and spark was jumping between contacts. When a spark is spread out over more then one cylinder, it doesn't fire correctly and that is causing it to bog out. Simple and a lot cheaper then all the other fixes I tried. Crank sensor was my next option though.

Replace the coil packs.....