This page provides information about the Blend Mtl in V-Ray for Blender.
Overview
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 element 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.
Parameters
+ Add - – Adds a new coat layer. Clicking on the 
button removes a coat layer.
Additive Mode – When enabled, the V-Ray Blend Mtl behaves like a multi-layered Additive material. The Additive Mode would often result 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.
Transparency – Specifies how transparent the Blend Material is. Black makes the material fully opaque and white makes it fully transparent.
BRDF 1
Weight 1