I would also use black. If the blocks are different sizes, I would lay them out and measure them. Then I would take the largest one as my designated size "master". Then sew black strips on all 4 sides of the rest of the blocks in wide enough strips to reach my "master" size on each block. Now that all of them are the same size, arrange them in a layout that pleases you, add black sashing and your orphan block quilt top is done!