DISCLAIMER: I did this with the Gimp, it may be different with any other editor. Also, this is my first tutorial so bear with me.
The method I chose is actually just tiles. By having a corner tile, a side tile, and a pure tile I managed to put together something that looked terrain-ish. (If you haven't seen the map I demoed a while ago look here
These are the textures I started with:
Put both of your starting textures onto separate layers, and lock them both. Hide the layer that is on top, as it will be a reference and will not be altered. Duplicate the top layer, and unhide the copy. This copy will be the blending. Add a new channel to it, and apply a linear, left to right, black to white gradient on it. Convert the channel into a selection, and use the eraser/whatever tool to. If all goes well, you should have a left to right blended texture, that is similar to
Save a flattened copy of that image in a jpg,tga,png,etc.
Now to do a corner. The method is almost exactly the same. Again starting with a duplicated layer you add a new channel. This time give it a linear square gradient that goes from one corner to another corner in black and white. Once again, use the eraser to remove the selected pixels and you should end up with something like
Once you feel you are done you can save a flattened copy in the format of your choice.
Once you have all of these textures you are done and can apply them to the map. I recommend curves since they are easier to texture than brushes.