I did some more sanding on the blade yesterday, and now 3 out of 4 bevels are clean to 100 grit.
I've noticed that using the Nick Wheeler method of sanding is causing the blade to warp, almost certainly because I am putting more pressure on the blade with each stroke.
I have not been using the hand sanding jig he demonstrated, but now I think I have to stop and spend time making one to avoid this problem. I have actually conceived a simpler (albeit less functional) version that I'll go with first. Basically, my version will be a 12 inch long section of very thick L-bracket with no second surface or set screws. Small C-clamps will hold the blade in place at the tang, and the rest of the piece will simply provide support along the length of the blade. I'll snap a picture once I've got it set up.