Dick Arentz describes this problem in his book "Platinum and Palladium Printing"

He calls it "The Black Plague"

He infers that it may be caused by defects in the paper stock, foreign matter in the coating brush, or particles in the coating material, such as metal salt precipitates or other contaminants. But he also infers that it is a fairly mysterious problem, and that while these are the most likely causes, the spots can still appear at random.

I recommend this book to anybody who is interested in platinum printing.