I would be worried that a tapestry fabric would be too heavy. The string, which operate the blind, could’t support the blind.

You just make an all new shade, if youre that ambitious about sewing. Shades don’t get ”slipcovered”.

Covering an existing shade with a thick tapestry is not a good idea. It's not that much easier than just sewing a new shade. You may need more than 3 cords to pull up the weight and the extra layers won't drape well when it is up.

Finding fabric that doesn't need to be seamed to get the width may be a challenge. If you do need a seam, you will need to match the pattern, which depending on the pattern, may require a lot of material.

There are good tutorials online for making roman shades and drapes. As sewing projects go it's fairly easy - all straight lines. If you need to put in a seam and have a fabric with a very large repeat you are going to need a lot of fabric in order match the pattern. A tapestry sounds like a very large repeat! If it's not *quite* wide enough you could put a border on either side of the tapestry to make it wide enough. That might be substantially easier (and cheaper!) that a tricky in matching the pattern.