Without seeing the fabrics, I would be thinking of maybe trying a grey for the bobbin and maybe for the top too if the thread was thin enough. I usually use Superior's So Fine 50 weight poly, which mostly blends in pretty well. Most of the time I really don't care for the look of a light thread on a solid black color. Another combination I have seen people use is Superior's Bottom Line in the bobbin with So Fine in the needle. Bottom Line is a 60 weight poly thread that really mostly blends in so what you see is the texture of the quilting not the thread color. Using a color other then either black or white just sort of is a compromise and gives a softer look to the quilting in my opinion. It really depends on your taste.