I use different color threads a lot, as I prefer to use pre wound bobbins and therefore have a limited selection of colors for bobbin thread. but I do teand to choose something that is fairly close to the same value as the top thread. so, it doesn't make for obvious pokies. however, you may have more difficulty with black and white as the pokies would be more apparant. have you thought about using a light gray on the top and a dark gray on the bottom? both would blend to the fabric fairly well and also then blend with each other. just a thought. or if you insist on the black and white threads, you could consider adding an extra layer of batting. I find that the thicker the quilt, the less pokies, since there is more material to absorb where that pokie is and it is more likely to land in the middle of the quilt.