This is related to
NMM and Sky Earth techniques. Those are often quite technical, but on blades there is a sort of a cheat which often looks good and isn't too difficult. If you paint one edge of the blade light, you do the other dark, so they are always opposite and contrasting. As one side blends light to dark, the other blends dark to light. Like this:
1: Plan out which areas are going to be light and dark. Each side of the blade should be the inverse of the other. (I suggest having the top side lighter, as it is reflecting the sky)
2: Start blending light and dark along the edges of the blade. You can do this by wet blending or layering or glazing (whichever technique you feel most comfortable with).
3: This is the finished blade all blended out.
4: You can blend different colours, or glaze colours over the gray to get different effects.
