Hi, currently you would need each material to directly reference its neighbors and then perform the blending using some kind of mask. There is no built in feature that can perform this blending.
One way is by using material functions for your layers, to make it easier to reference them in the same material. Then I would either use a texture mask, or build in some vertex tessellation around each square that has some alpha masking for each edge. Then you need to make a permutation for each blend and its neighbor combinations (manually), otherwise you can reference all 9 material functions directly in one big shader.