You all may know and use some fashion of this--but if not, its just a tip from a newbie that it works for. I don't have a belt sander and found this is one way to work on the finish.
I have a block of the dense, black, closed cell foam insulation (armaflex), about 2" wide x3" high x 4" long-cut it dead center on the 2" about half way thru and full length. just take your sand cloth/paper (sized to fit the full length/depth of your cut-when folded in half) slip it in with the abrasive sides facing and slide in the blade. hold the block and slide the blade / hold the blade and slide the block - edge up or down, changing grits as you go --- just like your using a saw.
give it a try--azmike

I have several sanding blocks for bladework.

For blades where the lines need to be maintained they are hard wood, generally cherry as that is what I have scrap from.

For blades with more of a "primal" look I have a block with a bit of leather glued to it from an old soft leather coat. This will soften out lines/hammer ding edges etc.

I have a piece of long side angle iron that fits in my postvise that I can then c-clamp the tang to for filing and sanding.

If you do a lot of thin flexible blades you will need a support for the underside when working them to keep them from bending away from the file or block.

Thomas

Here is what I use to hold the blade for sanding.

To Hold the sand paper , I agree with Thomas, use the right block for the job. I use lots of different blocks, running from rubber to plastic, and wooden paint stir sticks to polished Steel barstock, round or square soft or hard. I find that a sanding block foot long or so wrapped with paper in the middle, can be used 2 handed and really works fast.

Jens

I use wet/dry paper in sheets and use a mild steel bar. I wrap the whole sheet on the bar. As I wear out a surface I tear it off or fold it up on top.
I clamp the blade to wood (2X4) in the post vice and shim under the blade tapper with cardborad or paper.
The mild steel bar is 3/16 X 1 1/2 X longer than the sheet where I can hold each end at the same time.I use it across the blade and the length as I go through the grts.
The rounded edges of the bar make a good raidus for where the blade meets the racasso.

When I dont feel the paper bite I move to a fresh spot.
I only go up to 600grt thats the look I like
Ron