How do you turn off terrain blending?

If you drop your Terrain brush down to a single square (a "terrain map pixel") that gives you the size of the smallest blend. If each of your red lines is the center of 2 touching blue squares, then you need to reduce the square size. If it is several squares, use a sharper brush.

To change brush pixel size: Terrain->Resolution->ControlTextureRes. Doubling it cuts the pixels in half (and file size by x4.)

The other common trick (you see it in slightly older MMOs) is to use grass and other stuff to sort of hide the bad areas.

For the spec map, just a guess, give every other texture a "do nothing" bump map with pure black for the alpha (Unity likes to use alpha for the spec map.)