Why can't control strength of vertex color in blend weight mode of vertex paint brush setting?

In vertex painting, there are two mode “color” and “blend weight”.
When I select “color” mode, strength(in brush setting) works very well but in “blend weight” mode, strength is always 1.0 even though I set the strength value to 0.0. I want to control vertex color strength as I select “color” mode.
Is that bug or do I miss something?