this is a very difficult and time consuming project, you will have to remove most of the dashboard and the right front fender to get to the heater core, unless you have the time and handyness to do this i would suggest taking it to a mechanic.
It is a big pain in the behind. I don't know who designed the 94 Blazer but it seems that they deliberately made it to where you will have to take it to a mechanic to fix it. I replaced mine and you have to take off so much stuff, pull the dash out, prop it up with something to hold the dash up and you will bust your knuckles trying to get all the screws and bolts out. I put the core cover back on and just put one screw in to hold it on because there are a bunch of screws you have to take off. Cheapest estimate I could find was about $250 dollars to do it.