Ideas to blend materials of instanced static meshes

Does anyone know how I could blend the materials on the grass and mud? Currently they’re just made up of two sets of HISMs. I think I could make a tile set of instanced static models use bitmasking to place the correct models. But the result would be kind of blocky and I’d love it to be blended slightly randomly somehow. its hard to come up with a solution that would be performant, considering there are 65000 ground tiles.

I’m just looking for any idea you might have!