Ron,
Semichrome will take the blue off if it's a cold blue. If it's a salt blue the white cloth will be balck. If it's a rust blue the white cloth will turn brown. This is the first test I do on all Imperial or Weimar TD tools before I even think of measureing. Their is a good thread about this topic on the other forum. I hope this helps.
http://luger.gunboards.com/topic.asp?TOPIC_ID=12046