Hello and welcome to Just answer,
Below is a schematic for the fuel pump circuit for your blazer, first checks are ECM B fuse in the underhood fuse block for voltage both sides and fuel pump relay for click on an ignition cycle from off to run.
When the ignition switch is turned ON, the VCM turns the in-tank fuel pump ON. The in-tank fuel pump remains ON as long as the engine is cranking or running and the VCM is receiving the distributor reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was turned ON or if the engine stops. The VCM will also turn ON the fuel pump for 2 seconds when the ignition is turned to the OFF position.
no ecm voltage at fuel pump(purple wire)
The purple wire is for the fuel pump gauge, the grey wire is voltage for the pump.
does the relay click on an ignition cycle?
Does the ECMB fuse have voltage both sides?
Below is gauge schematics
In your original post you stated pump will run if wired direct, if direct wiring means providing a power and ground... then we have just proved that the vehicle is supplying the power now we need to check the ground.
Next step is to check the ground wire.
On the fuel pump connector connect a voltmeter lead on the gray wire and the other lead on the black wire.
The fuel pump grounds at G402 and is connected there via a splice pack SP423. these locations are for the 4door blazer if yours is 2 door let me know.
Near top center of second cross-member from rear, with fuel filler neck ground strap
notice the arrows
Then the problem is the connection at the pump.
check to make sure terminals inside fuel pump connector are tight. these may have spread or burnt causing a bad connection.
And was the meter placed on the solid black wire(C) and not the black with white stripe(D)?
Fuel pump connector.
Fuel Gauge Sensor Signal
Fuel Pump Motor Feed
that worked thank you very much would never of thought of that.
Thank you glad i could help
Good luck with the blazer