Are you using a steel ruler? I played Sherlock a few years ago and
prevented a suicide over black spots. It turned out that the razor was
trimming fine particles of ferrous metal off the ruler. The ferrous metal
particles were actually too small to see, but they attract the platinum
during development and turn into little black spots.
This was confirmed by testing and it was real. No sh*t.
This could be your problem.
Bostick & Sullivan
PO Box 16639, Santa Fe
505-474-0890 FAX 505-474-2857