I've been having an issue with my Blazer recently, every time I try to start it it's RPM's are 120-140 with the starting motor running, and increases to 340-400 after I let go of the key. It won't start during this process, and the only way I can get it to start at all lately is by turning the key and leaving it held until the engines RPMs increases to its natural starting point (this can sometimes take 10 minutes and it will stay at 120 RPM until it randomly jumps up) I assume that the fuel is making it to the engine, as I can smell it starting to flood after the first 30 seconds, and it sounds like it wants to start during the 340 RPM phase, but never does.

 

How many miles since the engine was tuned up, (spark plugs, spark plug wires, distributor cap, & rotor) using AC Delco parts? Have you checked fuel pressure and leakdown with a fuel pressure tester?