What makes gas leak into the oil on a Chevy Blazer 4.3 liter engine?

If this is an older vehicle, with carburetion, a ruptured diaphragm in the fuel pump could cause this.

If this is a fuel injected engine, these engines use plastic fuel lines under the upper intake manifold. These lines become brittle with age and sometimes leak causing fuel to run down into the crankcase. If this happens you have to replace the lines, relatively cheap and easy fix.