Montblanc's blue-black ink contains iron gall and is document-proof: soak a written sheet in water and the blue dyes wash out, while the black iron-gall compound stays put. It can be used in piston fillers without hesitation, as long as it is never left to dry in. Before a longer break the ink should be emptied out and the pen flushed with clean water until the water no longer discolours; a flagging ink flow is likewise a sign that cleaning is due.
Mixing is not to be recommended. An experiment in equal parts with an ordinary blue ink did give the colour wanted, but the mixture congealed on the paper like jelly and the document-proof quality was lost: the dyes washed out completely. In the ink reservoir, and particularly in one that is nearly empty, such a mixture can turn highly viscous. That document-proof inks lose that property as soon as they are mixed with ordinary ink holds for other brands as well.