I use a very fine "running" stitch to piece, but it can depend on the type of piecing you are doing., and I've heard folks call different stitching, blind stitching.. I think the first thing would be to make sure we're talking about the same stitch.. but bottom line, if you pull hard enough on hand pieced 'pieces' there will be gaps. whether they are blind stitched or running stitched. size of the gaps depends on the size of the stitches.
So please tell me what you are calling "blind stitching."