I am not that sure the harmful PM would be detected by the human eye when cleaning an air filter. That dark stuff must be simply dust. On top of that, filters should be replaced and not really cleaned.
In my machine I have an embedded PM reader which rarely gives a reading above 30. When the filter was new I could go below 10, but now that the filter has been at 70% usage I don't really manage to go below 20. To note I don't keep the window open in my bedroom.
In the rare occurrence I have somebody smoking cigarettes at home, the reading goes above 100 almost instantly!