In my experience, if you first heath the blade in scalding water and then submerge the blade in vinegar, you'll get a solid, evenly colored darker blade. Let it soak for an hour or so. If you want it darker, repeat. For some reaxon, some of the darker color will come off if you then polish the blade lightly with Flitz.

I've done some kukuris that were too large to submerge in anything I had so, after heating the blade, I wrapped the blade in vinegar soaked paper towel and let it set in one of my wife's baking dishes. (She loves it when I do that.) Got kind of a tie-dyed effect.