Hi Daniel,
There isn’t currently a way to read the blend weight directly in a material as the intended usage is to blend between a base material and it’s instance. There is a workaround you could use that takes advantage of this system however, by creating a base material with a scalar parameter set to 0, for example.:
Then creating an instance where the BlendWeight parameter in this example is set to 1 instead. Using the material instance as the blendable will cause the blend weight UI to interpolate the parameters between 0 and 1, effectively giving you the alpha you’re after. In the above example the screen would fade to red with the slider.
Thanks,
Chris