I've used a couple of methods to blending when creating dungeon tiles.
A) Fade: Great for when you want complex textures to blend into each other.
B) Hard edges: Great for when you connection are always going to be the same (such as equal width tunnels or actual tiled floors). Makes JPEG an option for good load times in a VTT.
C) Both: Two great flavors that taste great together.
TilesExample.jpg