How can I Make a Cycles material with gradual gradient fill? I would like to use a material to create a blending effect from the edge of geometry to the mid point like I can with the Erode Node in the compositor. I don't know how to get edge to edge distances, but I feel like it could be hiding in pointyness attribute? I don't want to paint it or preprocess it (otherwise I'd stay with compositor)

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You are right that using pointiness will do what you want. Take a look at the following node setup. Changing the subsurf modifier divisions changes the amount of blending (as well as adjusting the color ramp)

In short, pointiness is connected to the factor of a color ramp and that to the diffuse shader's color.