I could not found how the blending of individual textures in multitextures is defined. Shall textures be added, multiplied, colours replaced based on alpha value or something other?
P.S. Nothing in the specification gives a hint that multitextures is supported at all. If there was no example file with a multitexture, I would have never found it myself.
It is my impression that COLLADA is currently focused on programmable shader-based materials. Therefore that blending information is contained within the <program>, etc. It merely references the textures declared earlier.
I am currently working on a “fixed function” profile that will address the older fixed-function pipelines as another profile technique.