Long story short. Changed starter, changed alternator. Battery is good. When key is the on position it loses all power and won't crank. Can jump start. Where am I probably losing power? Everything I tested is good.

 

How was the battery tested to be good? (Load test or just voltage?)
Any fuses blown?

 

Load test at 9.8. Didn't trust my Chicago load tester and took it to auto zone and passed there also.

 

We would probably need more information about the Blazer and the symptoms.

What Blazer, what year, which engine version?
What has been modified? (Repairs, modifications, anti theft?)

When you open the door, do the courtesy lights go on?
Without the key in the lock which appliances work? (Lights radio?) This will depend on your Blazer. Refer to manual which should work i.e. hazard flashers, brake lights.
With the key in the lock, turn to ON, then return to the OFF position. Again, what works, what not? What should work?
With the key in the lock, turn to ON - not to CRANK and check again, what works, what not? In this position nearly everything should be working. Internal and external lights, flashers, etc.
How does the instrument cluster behave in all these positions?
Have you checked "all" fuses? Including the high amp ones? (Blazer's have several locations to check)
Are there stored fault codes?

We see so many electrical problems which are related to wiring which is either broken, eaten, burnt and bad repairs and modifications that it's always worth looking into that first.