car starts up ok,smoke color is black.
,but clears up as it's driven.
Soot in exhaust pipe at rear is about a 1/16 deep.

Replace the ECTS and connector with gm genuine parts. You are likely running way rich... more than intended when cold. When the ects goes bad it usually goes open circuit which corresponds to the ECM interpreting the coolant temp as -40 ish. Yours may not be open circuit but if there is corrosion or poor electrical connections, the circuit resistance increases and is interpreted as the coolant temp being cooler than it really is. air/fuel skewed rich. If you had an injector stuck open I don't think the issue would clear up as you.drive. and the ects pretty much never throws a code b c -40 is sill considered within normal operating range.

If your pre cat was plugged, I'd think you would have significant performance issues.