my 2001 blazer wont start.

Make sure battery has a full charge and connections all good. Note the diagram, I see about 4 fuses, some hot all the time, the others goes hot with key on. You can use test light to check voltage at fuses. If you think the starter motor has a problem, you can remove it, lay it on the ground, hook up jumper cables properly from battery to starter motor, at the least, it should kick out the starter drive gear and spin, if it won't even do that, I suspect the starter motor. Another thought might be the neutral switch for automatic. In the diagram, two voltage circuits at starter motor, one comes from the battery, hot all the time, The S terminal goes hot when relay is energized. The starter motor should ground through engine block.

I've replaced the neutral safety switch and the ignition switch with wiring harness.the starter and battery are both good. but sometimes i turn the key and everything comes on but the starter doesn't engage. if i keep turning the key from ac to start numerous times eventually it starts

You have to test when problem is ongoing. Check voltage and ground for starter motor when problem is ongoing. Any testing, make sure transmission is in park or neutral and parking brake is set. Make sure vehicle can't move.