If I were going to do 2 colors, I would do the black areas in one pass, and then go back and do the colored area and only then advance my quilt to the next section. I float all my tops including king size and find it far easier than any other method and I have had zero issues. I agree that black on black will be difficult to see when quilting, and in the end I always weigh effort and results. It’s a betuful top and I can’t wait to see it all completed and welcome to the wonderful world of Longarming, it really is a lot of fun.