Thanks for the info. My experience here is a bit different, but may lead to a third theory that readers can use. I printed Pt/Pd for a couple of years with no "black spot" problems whatsoever. Recently, I started having all kinds of spot problems (mostly small spots). I was advised by B&S that I may have developed a microscopic burr on my rotary trimmer that was allowing small metal shavings to be deposited on the paper. The paper I was using was from a batch I've used before with no problems, so the explanation sounded intrigueing/reasonable.
They suggested tearing the paper instead of cutting it on the trimmer. After a couple of less that perfect tearing attempts, what I did instead is aggressively brush the surface of the paper with a soft brush. This has worked for me and the problem has not returned.