Cycles multi material and smooth transitions?

Hello,

I have a simple cube with one loop cut in the middle. Half of the vertices belong to one vertex group called blue.
The other half belongs to the vertex group called red.
Each vertex group gets its own material blue and red. The renderer is cycles.
If I render now there is no smooth transition between red and blue, there is a sharp cut between the blue and the red vertex group. How can I get a smooth transition / fading between blue and red?
I tried to use weight paint but it seems that cycles ignores the weight, it’s still this sharp cut.

Thanks for any idea!

If you want to use vertex paint here’s one option


or another option


Hi Richard,

thanks for your quick reply, but I need it for vertex groups, since the same vertex groups I use e.g. for displacement maps and both must match.
Using vertex color doubles my work since I have to create vertex color and vertex groups / weight paint.
So I prefer to have vertex groups and weight painting. Is that possible?
Is there any material node which uses the weight paint of an object to influence the material?

Thanks again!