I agree with the answers here.
I would certainly guess its to do with the way it is gripped and feels in the palm of his hand, which would then open the question as to why it is gripped that way with all racquets with the black edging facing upwards?
Personally, I put matt black tape around the edge of my racquet head, to protect from scratches but also so that when I catch sight of my racquet out the corner of my eye I dont get any glare from it, so it could be for similar optical reasons. Looking down at matt black is calmer than looking at red.
I do have a slight OCD as well though, in that I always have the Prince writing facing up on the throat of my racquet and the knot on my rubber band dampener on my backhand.
"The older I get....the better I was"