Blends between 2 materials are possible using a layeredShader. To use a blend map between the two, connect a file texture to the layeredShader's first child node's transparency attribute. (or to the Blend Factor attribute if you're using an indigoShader).
Blending with nulls is not possbile at present because there's no "null" type in indigoShader yet. This should be quite easy to implement, I'll do it soon
