We have published the first video tutorial on the Alpha blend node in our series of video tutorials.
In this tutorial, we create a scene with two terrains - a Slope and a Perlin Noise - and then apply a Disk mask. Double clicking on the Disk mask opens its properties; you can see as we start playing with the Radius and the Internal Radius parameters, the changes are seen in real time.
We also see how the Alpha Blend node works in little more complex project. http://ow.ly/WjLx30i0RXh (http://ow.ly/WjLx30i0RXh)
(http://www.wysilab.com/forum/images/tuto_alphablend.PNG)