So I finally found some almost-matching purple yarn (the blanket is a variegated blue/green/purple) to fix the seam. Since the new seam yarn is a little thinner than the original seam yarn, I took out the whole strand of the old seam yarn, which left about 1/2 of the seam undone between two panels (center to edge of the blanket). That seam was sewn up with 2 strands - one from each edge to the center.
I started at the center and started seaming up the blanket. It went easily even though some of the edge stitches were a bit stretched out, but nothing too bad. I made sure to line up the 12-row rectangles to make the seam as neat as possible. One rectangle didn't line up and after seaming/ripping/reseamimg 3 times, I realized that one panel had a 12 row rectangle and one panel had an 11 row rectangle. No problem, a little fudging and it was all set.
Then I get to the edge of the blanket. I was off by an ENTIRE 12 row rectangle. How the ???? did I mis-align that badly? I did have the blanket folded up in my lap while I was seaming (yes, I checked a few times to be sure I wasn't sewing the blanket shut
So I counted rectangles in both panels. One panel has 24 rectangles. One has 25 rectangles. Aha. Needs a lot of fudging. I tried unseaming back 4 rectangles and fudging back to the edge, but that's too much fudge in not-enough space. I'll be unseaming most of what I seamed up yesterday and re-seam with fudging so the 2 panels meet at the edge.
And it was such a nice seam I did. Oh well, a little seaming practice never hurt anybody.
I did a mattress-stitch seam, and the original seam looks like it was a whip stitch seam so my seam and the original seam look, and lay, different, I haven't decided if I'll undo the rest of the original seams and re-seam it all to match or leave it mis-matched. A lot of that decision depends how secure the rest of the seams are, and I haven't checked that yet.