I use one color in the combination to act as the "blending" ink. For instance, in the first combination below I used Tumbled Glass to do the overall blending. I always use the lightest color as the "blending" ink. I sort of look at this color as the boss that blends everyone together, so to speak.
Tumbled Glass - "Blending Ink"
You can find all my Distress Ink Blends here.