Part of the problem of black and white is that we can't see color.
I honestly think the axe had seastone in it. Seastone =/= devil fruit power, and immortality is conferred by devil fruit power (Trafalgar Law's ability), so negating the power negates the immortality. We don't know that this is haki at all. But because it hasn't been explained yet, they might draw it as such.
