User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-84.197.239.101-20160926192137/@comment-7418318-20160927132626

Bloodman said he had all the curses of the Nine Demon Gates and we saw him use Ice Devil Slaying, which most likely means that ISD"M" isn't magic but rather a curse.

And as we know from Kyouka, curses work against stuff that's immune to magic, like magic sealing stone, so there's a good chance that Natsu's magic immunity (if that's what it is) would only work against Gray's regular Ice-Make, but not against Ice Devil Slaying.