Just to cover all the "free" stuff: check on vacuum leaks from rotten hoses or connectors. All air passing the metering unit and entering the engine after that can cause a lean condition.

Thinking of this i wonder if we always see the misfires at more or less the same cylinders. Not that this could be related to the location of the vacuum ports in the plenum?

 

Check cam sensor retard and/or wiggle the rotor. Probably worn distributor gear. Either will easily tell you. Cam sensor retard value should optimally be zero +\- 2 degrees. This is the most likely culprit on a 2nd gen Blazer.

Newblzrowner and erinvarghese
are on the right track. 2 solutions for same problem. Replacing the gear or entire distributor is better choice as gear will continue to wear - but either way cam sensor retard (where the rotor points when the spark comes through distributor) should be optimized for best performance. Reading Cam Sensor Retard requires a capable scanner plugged into the OBD2 port. If you are interested you can set one up on your Android phone with a Bluetooth connection for around $35-$40.

IMHO don't waste your money on O2 sensors at this point. Those and the crank sensor and MAF sensor are the most often shotgunned parts that seldom fix a problem.