Layer blend modes let you combine and display the content of one or more paint layers in combination and change the visual characteristics of the final composite paint layer (texture map) that results. Layer blend modes work in a manner similar to other popular image editing applications.
Layer blend modes do not change the image data on the paint layers so are useful for non-destructive modification of the paint layers.
To set blending modes for layers:
When a blend mode is set for an individual layer, the effect of the layer blend mode is updated on the material in the 3D View.