I don't use vray, but this behavior makes sense. When you change opacity of the blue material the blend node is using that like a layer, so you get more grey material as a result. You'll want to add another node after the blend for total opacity, or find a different way to blend the two.
Maya. I use vraymtl with grey color as "base" material for vray blend node and vraymtl with blue color for "coat" of the same blend node. And as a "blend amount" i use black and white texture. Everything works as expected until i want change blue and have it transparent and not blue . Blue becomes white and not…