I tried to start my 89 Blazer with a 4.3 and only get one click coming from starter. Replaced starter and have a brand new battery and still just a single click when turn the key. Amp meter drops to 8amps or less and nothing. Starter ignition starter relay? Ignition relay?

 

Ground connections, Connecting cables. In addition to the relay you already mentioned.
Normally the ignition relays are not related to the starter. It's really pretty simple on the 89's. If it clicks seems that the ignition key is working properly but the starter relays may be the issue.

Depending on how good you are with car electrics jumping the starter motor directly could be an option to rule out the relay. But it involves high Amperes and you need to know exactly what you are doing in order not to fry the starter, blow the battery or fry your electronics.

 

Single click at the starter is usually the starting solenoid engaging so relays are working as they should - the voltage drop is because the starter can't spin the engine - did you happen to check and see if engine will turn by hand and is not seized up .....