Blazer wont start unless jumped

Use a digital multimeter across battery posts, what is the reading with engine cranking, no jumpers. The generic specs says it shouldn't drop below 9.6 volts. Cranking is starter motor action, not starting. Not only make sure battery has a full charge, make sure battery connections are all good, no voltage drops. With vehicle cranking, the systems get battery voltage, with vehicle running, the systems get alternator voltage, can be several volt difference, if charging system is working properly.

If it cranks good but won't start, have a helper crank it while you check for spark at the plugs, if spark everywhere, use a gage and check proper fuel pressure and check fuel injector pulse. Any applicable trouble codes?

I can't use what somebody else told you. If it cranks good but won't start, do you have good spark everywhere, if so do you have proper fuel pressure and injector pulse. You need to do some testing. Ok, so the battery and charging system are working great, why do you need the jumpers? Because of a voltage drop issue or a component that needs a little extra kick in the britches. My first thought would be the fuel pump, that is just a guess. If you have a spider fuel system, that system is noted for problems.

Please let us know what you find so it will help others that come to the site