My 1999 Blazer (with 4.3L) won't star after a significant rain. It will crank, but not fire. I replaced the cap and rotor, thinking it would fix the problem. It didn't. Any suggestions? Thankyou in advance!

With one sparkplug wire disconnected & a known good sparkplug connected to it and grounded, have an assistant try to start the motor and see if the sparkplug makes a spark.
If it sparks then I'd suspect bad/grounded sparkplug wires.
If there's no spark then I'd suspect a bad ignition coil and or bad/grounded sparkplug wires...all help gratefully accepted.

It starts and runs fine when it dries out. Once I actuallys ste a small space heater under the hood to speed it up, and it worked. Should I still try the coil and/or wires?

Yes, if the wires have any cracks and any road salt splashed on them, the wet salt conducts electricity and shorts it to the closest ground. Same goes for the ignition coil.
A remote possibility is the Crankshaft Position Sensor (CKP) wiring connection.
The CKP is mounted just behind the harmonic balancer at the lower front of the motor...all help gratefully accepted.