I think the problem may be that the fake stones are 'grown' from one tiny position. So even if you blend your grasses by them, they will still grow just besides these tiny positions, hence through the stones. If you get my point. Perhaps if you use a PF to distribute your fake stones that (inverted) would be the ideal blender for the grasses? Perhaps slightly change the color to get more black (=less grass).
---Dune
