Bryan, I've turned several different pens using Caribbean Swirl and you'll probably need to paint the tubes a light blue color. I've done all mine this way and had no problem whatsoever. I think blackening the tubes would darken the color of the blank and change the color from the Aqua like color to a much darker blue IMHO. I turn a lot of acrylics and celluloids as well as homebrew resins and paint all my tubes a color similar to whatever the color of the blank is