A patch will show more than vertical seam. I would take out the first three inches of stitching on that block on both sides, add a strip sewn nice and straight and press it well. Resew the top and bottom of the block. Once it's quilted, even you won't be able to find it. I assume you have a strip of that fabric 3 X12 or whatever the block size is. Make sure the grain is running in the same direction. Oh, wash the strip first so that matches too, as you have washed the top. Good luck. As a friend used to say, "It won't show on a trotting horse."