One more question

These materials have two textures that can blend into eachother using the "Paint Alpha" thingie on displacements.

Hello.

This effect is created through use of displacements. Displacements allow geometry to effectively painted on (alpha painting), or molded into shapes. Displacements are commonly used to create terrain in maps and other effects, through its ability to be changed in shape. Here is an example of a displacement:

Displacements, in addition, as said before, have another common use: Alpha Painting. Alpha painting is where a texture applied to a displacement can be "painted on" using another texture. Here is an example, similar to you have shown in your original post:

The ground texture shown in the screenshot above is a displacement. Notice of how at certain spots There is a coal-like texture applied and a flat, ground texture applied. This is an example of alpha painting. However, only certain textures can work with this. Textures can be "loaded" with two textures, making them compatible with alpha painting. To know which textures are compatible, the difference is fairly obvious. In the texture browser, the thumbnail for the texture shown will be a gradient between two textures. Here is an example to better explain:

For tutorials on how to do this, a simple search on Youtube will explain everything. Hope this helps!

It's called a blend material. There is a dedicated shader for them.