How do I change or create a custom 'blend type' for the animation blend trees?

![alt text][1] I'm using a 2D blend type for my animations (this one is the '2D Freeform Cartesian'). Its almost perfect for what I want, but I don't want the animation highlighted in yellow to have any influence when the red dot is in this location. As you may know there are only three 2D options available. Does anyone know how I could change the influence/weight of these animations in the 2D blend trees, or create a custom 2D blend type? Thank you [1]: /storage/temp/127456-blendtype.jpg