I would block the quilt top. This involves a large flat surface you can pin into and spray starch or water. Sharon s chamber has some YouTube videos on blocking blocks, but you can also block a quilt top or even a finished quilt in a similar manner. Carpeting can work as your flat surface.
How did you measure for the borders? Did you measure through the center of the quilt each time? I don't think the problem is with the black border strips. I think it is the other borders that are causing the waviness. It looks as if you had to match the corners and that there is extra fabric in-between the corners that are creating fullness.
Last edited by Prism99; 01-07-2017 at 04:45 PM.