Janger and I were talking and he suggested I post this... 'Prussian Blue' as used in the machinery rebuilding and tool and die trades refers to a petroleum carrier paste containing more than 80% particles of ferric ferrocyanide. This is used to take measurements in the order of tenths of thousandths of an inch - primarily for fitting or scraping (there are other uses too).
It is often confused with Layout Blue or Layout Dye, which is often blue in colour and only comes in liquid (bottle or spray) form.