I've tried all kinds of search terms, and every program I've come across simply falls short of the most simple task I want performed.

Input Image A (Tile A in diagram.) Input image B (Tile D in diagram.) Pick Blend Mask. Output 12-tile tileset in the given format(Tiles B, C, and E in diagram) with blend mask transitions.

And that's all I'm asking for. I can resize the tiles myself later on, but I want to work with higher-resolution textures, starting at 128x128 and going no higher than say 512x512, and it seems no program wants to actually let me use something of that resolution. Doing it manually in GIMP or Photoshop? No thanks.

http://i.imgur.com/KLXPOwW.jpg

This site link pretty much gets right into the meat of what I'm trying to do (Smooth Transitions Between Regions section.) Is there not software that does this automatically?

http://devmag.org.za/2009/05/28/ getting-more-out-of-seamless-tiles/