V-Ray Blend Material can be used to layer several V-Ray-compatible materials in an efficient manner. It can be used to create complex materials like car paints, human skin (when Fast SSS2 material is used as a base material) etc.  

You can use the Material Select render channel to split the different sub-materials of the Blend Material into different render elements.  

V-Ray Blend Material takes a base material and applies other materials (coatings) on top of it. This works like a stack, where each coat material blends between its own shading and that of the materials below it in the stack.

This section lists input sockets that are not found in the Properties tab but only as inputs for the node.

Transparency – Specifies how transparent the Blend Material is. Black makes the material fully opaque, and white makes it fully transparent.

BRDF 1 – The base material over which other materials are layered. If this is not specified, the base material is assumed to be a perfectly transparent material. All following coat layers are numbered accordingly.

Weight – Specifies how much of the final result is contributed by the corresponding coating material and the rest of the materials below it. If the Weight is 1.0, the current material blocks the materials below it. If the Weight is 0.0, the current material has no effect on the final result. This parameter can also be controlled by connecting a texture node.

+ Add - – Adds a new coat layer. Clicking on the button removes a material layer.

Additive Mode – When enabled, the V-Ray Blend Mtl behaves like a multi-layered Additive material. The Additive Mode often results in a physically incorrect material (e.g., a material that reflects more light than falls on it). It is not recommended to use this option, unless you know what you are doing.