rockp2 is correct. Most years have three sensors and usually the one behind the cat is B1S2, except for 96/97 when the 4th sensor behind the cat is B1S3 and in 98 when there are three and the one behind the cat is B1S3. Also, the scanner nomenclature can be wrong. If you need to know for sure, pull the signal wire one by one and map the sensor PIDs in the scanner.

George

Thanks for all your help, it's clear you guys know what you're talking about. Thanks to all of you for your clues and advice. It seems conclusive that B1S2 is the one upstream from the cat (front of cat, towards engine) this is the one that previous owner supposedly replaced, it looks new. But there were a ton of things he neglected to divulge about the vehicle. Someone please SHOOT me if I look at or buy another vehicle on facebook marketplace, some good people there, but mostly LOSERS.
But getting back to my Blazer, it's a goodie I think, I don't want to pour a ton of money into it, but it's sweet and deserves some much needed attention. I'll keep you folks posted, and let you know what happens. Still, any more advice would be very much appreciated....I guess the consensus is, that B1S2 is the O2 sensor right at the front of the catalytic converter.......
Bill in VT

Dude you got a RUST FREE 90K 26 year old truck IN VERMONT...I'd say you're doing pretty good! My '98 is around 270K (I replaced engine at 219K) and has new panels I've had to weld in due to body rust.

As GeorgeLG stated, some of these cheaper scanners will give you the wrong sensor location as the problem. So check both cat sensors to see which heater isn't working. Possibly previous owner changed out a good sensor.

Again, thanks, rockp2! And everyone else...I stumbled upon a post here on BF, and found a previous post "97 o2 sensors" which showed the diagram of the sensors on the '96-'97, clearly showing B1S2 as the one right in front of the cat. That IS the one that was replaced (supposedly) but because of all the BS the seller gave me, I can't ascertain he replaced it with the CORRECT sensor. After the snow dwindles, I'll get outside and check it out, and let all of you know what I found (or fixed). This poor Blazer was really abused while being stored in a garage for many years, showing a lot of small dings and scratches from things being stored on it and against it. But the '96 wants another chance, my wife really likes it too. It does really well in the snow, even in 2WD, with the new Goodyear Wrangler's. I hope I don't need to replace the heater core, it gurgles sometimes, and occasionally has a slight odor of anti freeze, I think (from what I've read, it's not a fun job). I'm pretty much a GM guy, don't know why I haven't discovered these Blazer's before.....

Oh yeah, by the way the radio CD player is pretty much dead, until it occasionally springs to life...does anyone know if these have a separate amp? LMK
Thanks Bill

Some DO have an amp, most don't, it depends on the RPO code. Even the Bose sound systems come with AND without (mine is without on a Trailblazer) but there's options at Crutchfield to throw an aftermarket head unit in and keep the door chimes, etc. either way. At this point, the factory paper speakers are probably torn/rotted or soon to be torn/rotted and pretty much anything is an upgrade! I think I threw some $25-30/pr Planet Audio 6 1/2" speakers all around with a 19 y/o 10" sub and amp (a Valentines day gift in 04 to then GF, now wife that keeps moving from vehicle to vehicle) in the very back and it's surprisingly good. As for the O2 sensor, one thing you'll quickly realize on these blazers is: They like AC Delco and Delphi and it's a crapshoot with everything else when it comes to anything electrical that has to talk to the ECU.