William, I don't have the Arc Blast, but I do have the Exo and Zero, which I believe have the same hip belt.
The hip belt is cuben hybrid, not pure cuben. I haven't found it to be too slick, but it probably depends on what you're wearing. I, too, have narrow hips and found that the belt on the didn't connect too well at the outside of the back (my frame isn't broad enough). On the Zero, I got Joe to attach the belt an inch more towards the center of the back and it fits great, nice and snug. I also got both belts made a little shorter than the standard short belt as I have had other packs where I couldn't cinch the belt tight enough.
Hope this helps.