A bit of a cheat with power weapons! I used Daler Rowney FW ink Payne's Grey and White, but you can substitute any white or black ink, or even highly pigmented paint. I'll add step-by-step photos at a later point.

1. Basecoat the entire blade Silver or Stormhost Silver
2. Add spots of White and Black opposite one another alternating down the blade - Three pairs will look best on a normal 28mm sword. (as I said above, I used White and Payne's Grey from Daler Rowney)
3. Before the spots dry, smooth them out so you get a gradient from the silver up to the white and down to the black, but leave pure silver showing between.
4. Thin down the black ink 1:3 with water and glaze from near the centre of the white to the black, leaving a little pure white visible. The direction of your brush strokes is important - always move from lightest to darkest, as that's where the brush will deposit the most ink.
5. If you're happy with the blend, leave it here, but if you want it smoother, repeat step 4.
6. Glaze over the whole blade with a 1:1 of Turquoise and water. You could also use Guilliman Blue