Would there be a chance the rear leaf springs from a 1st Gen 4 DR 4x4 Oldsmobile Bravda direct fit on a 2nd Gen 2000 Chevy Blazer 4Dr 4x4?

 

It's possible, but you'd need to measure it to know. Measure from the center pin out to each end, and then from end to end.

That being said, I'd worry that a leaf from a first gen truck would be much more worn out and saggy.

Another option if your leaf pack is saggy but not broken is that it's quite common to take the main leaf from another truck, cut off the eyes and use it as an add-a-leaf in your original main leaf pack directly under and supporting the main leaf. This will stiffen up and address the leaf sag without making the ride harsh.