I have a 1995 Blazer that has a knock when started, but the nock isnt in the motor, its closer to the core support. It stops when the vehicle gets warm. Any ideas?

 

Cause the description sounds like a definate engine knock.Without hearing it myself i possibly could be something else like something loose.These engines are notorious for spinning rod bearings.
Depending on where your engine was built at either Romulus MI or Tanawand New York one of the factories didnt follow very close tolerances when manufacturing this engine.This is just my opinon but Im not so sure GMs balance shaft idea doesnt have something to do with it.
Which gives me another idea about your problem,These balance shafts haveball bearings on each end of the block where they are inserted.Its possible thats where your noise is coming from.You can get an engine stethascope and put it in the area where you here the noise is coming from.Then you can pinpoint the location of the noise/knocking

 

Check your clutch fan if its wobbly thats where your noise could be coming from also.

 

could be a number of things, check your tensioner pully as well.

usually if its internal, it wont really stop once the engine is warm.

 

I have had many 4.3 engines with high mileage that develop a rod knock at a very cold start (-20* F) If allowed to warm up and driven gently I have found that one can usually get through the winter! I had a 250K engine that did this routinely and I was able to keep in service simply by fresh oil and gentle warmups! My current unit has 193K and I hear a slight knock upon starting in temps below 30* but it disapates after 30 seconds. Do I want to pull the pan? NO! Ill just keep it maintained-Chevys will run for a long time-even when not in the most perfect condition!My Jimmy will go another 50K easy...