Yea post process materials behave like this…
Even though they are in “blendables” and you can modify their “weight” it might not work if that is the only material there. If it is the only one, changing the weight just above zero will make it “the most dominant” and will set it at 100%
You can make a dynamic instance of the material and modify it there. If you are not sure how, I recently did an example here: trying to change a variable inside a post process material. - #7 by dZh0