Hi Eugene,
Thanks for the textures! You can download the plugin here:
http://www.worldviz.com/download/files/textureblend.zip
I included the sample mask image you sent me in the example. Let me know if this is okay with you.
The mask image you sent me didn't have any alpha channel in it. So the plugin doesn't use an alpha mask, it uses a grayscale image mask. The white areas of the image represent where the first texture will show up. The black areas of the image represent where the second texture will show up. The gray areas of the image represent the percentage to blend between the first and second texture. Look at the sample script and the readme file for more information.
Good luck!