But in your example you still don't know if it was your testing process that shed 100 plastic particles or if your distilling process shed 100 plastic particles, meaning you don't actually know if B was or was not the source of the plastic particles. Was it your testing process that introduced those 100 particles, was it the distilling process that introduced them, 50/50, or something else?

B would be inconclusive against what you'd hope to be some kind of background, as its not significantly more but one couldn't conclude the source didn't shed that 100 because you don't actually know if in the blank the 100 particles of contamination was definitely your testing process or the source material genuinely having 100 particles of contamination.

I do agree though, in the A case one could pretty easily conclude whatever generated that sample is adding way more particles than an attempt at a baseline/background.