Usually if you ask for "prussian blue", you will get something even from the autoparts stores where the counter help doesn't actually have any experience working on machinery. Last tube I bought was a Permatex brand product I believe.
I have used a soft black coating in aerosol cans that is sold for coating plows etc to stop rusting when not being used. It doesn't ever harden like regular paint and can easily be washed off with gasoline when you're through. It does take a little more resistance applied to the driven shaft to get it to make a good mark.
I've used white lead (used to be able to get it at hardware stores for plumbing) a few times but Prussian Blue wiped on fairly thin works about the best. Just about any automotive store will have it. .
I've heard it referred to as 'bearing blue'.
Daron