I really dont understand what you exactly mean with blending. A texture is bound to a mesh object. It ends at the border of the mesh object. If two meshes are close but have the same texture they will most likely not have seamless transitions if the map designer didnt align them the way you did. So when creating textures make sure they are periodically tileable and try to paint them the way the original was painted. Also you will most likely have to work over the normal map of the texture wich defines geometrical detail. Also some textures have specualar or even glow maps wich are all seperate dds files. You need to keep all that in mind. For example if you create a custom texture for lets say rocks but you replace them with something like bricks, you have to create a normal map for your bricks and replace the original rock normal map with it. If you dont do that it can look rather weird ingame. same goes for every other texture map.