This can be the result of a few things. An engine vacuum leak, dirty throttle body, or even the idle air control valve.
What I suggest is to first confirm there are no engine vacuum leaks. Here is a link showing how that is done:
Next, consider cleaning the throttle body / throttle plate. Here is a link showing how that is done:
If that still doesn't help, then I suspect it is related to the idle air control calve (IAC). The IAC is responsible for maintaining idle speed. Take a look through this link. It shows in general how one is serviced and the second shows general replacement.
Here are directions specific to your vehicle for replacement. The attached pics correlate with the directions.
2002 Chevy Truck S10/T10 Blazer 4WD V6-4.3L VIN W
Vehicle Powertrain Management Fuel Delivery and Air Induction Idle Speed/Throttle Actuator - Electronic Service and Repair Procedures
IDLE AIR CONTROL (IAC) VALVE REPLACEMENT
1. Turn OFF the ignition.
2. Remove the air cleaner outlet resonator.
3. Disconnect the idle air control (IAC) valve harness connector.
4. Remove the IAC valve mounting screws.
5. Remove the IAC valve and O-ring from the throttle body assembly.
CLEANING AND INSPECTION PROCEDURE
1. Clean the IAC valve O-ring sealing surface, pintle valve seat, and air passage.
Remove excess carbon deposits with a small amount of carburetor cleaner on a parts cleaning brush. Follow any instructions on the cleaner container regarding cleaner usage.
DO NOT use a cleaner containing an extremely powerful solvent such as methl ethyl ketone.
Shiny spots on the pintle and seat do not represent a bent or misaligned pintle shaft.
If heavy deposits are present in the air passage, remove the throttle body for a complete cleaning.
IMPORTANT: DO NOT reuse the original O-ring.
2. Replace the IAC valve O-ring.
IMPORTANT: Use an identical pan when installing a new idle air control (IAC) valve. The IAC valve pintle shape and dimensions are designed for specific applications.
1. On a new IAC valve, measure the distance between the IAC valve pintle and mounting flange. Use finger pressure to slowly retract the pintle if the distance is more than 28 mm (1 - 1/8 in).
2. Lubricate the IAC valve O-ring with clean engine oil.
3. Align the flange holes with the mounting holes on the throttle body assembly.
4. Insert the IAC valve through the O-ring into the throttle body assembly.
NOTE: Refer to Fastener Notice in Service Precautions.
IMPORTANT: The screws used to attach the IAC valve are coated with thread locking adhesive. Clean the attaching screw threads and apply thread locking material GM P/N 1052624 or equivalent.
5. Install the IAC valve mounting screws.
Tighten the screws to 3 N.m (27 lb in).
6. Connect the IAC valve harness connector.
7. Install the air cleaner outlet resonator.
8. Perform the IAC Reset procedure:
8.1. Turn ON the ignition, with the engine OFF, for 5 seconds.
8.2. Turn OFF the ignition for 10 seconds.
8.3. Start the engine and check for proper idle speed.
Let me know if this helps or if you have other questions.
Images (Click to enlarge)
Saturday, January 30th, 2021 AT 8:48 AM