Use a rubber roller (artists store) to transfer your blue onto surfaces. You really need to roll out the blue on a surface plate so you have a thin layer on the roller then apply to the mating part you "aren't" scraping. Is your granite surface a true surface or a chunk of worktop. Worktop is OK, but don't assume it to be as true as a surface plate, try to get it checked against a known surface, or all your parts will have the same defect....

Nothing wrong with HSS scrapers if the job isn't huge, you need to take your time and be gentle when you start. Short strokes, for me a 45 degree herring bone pattern worked on larger areas. Don't be afraid to recheck the work often to begin with. Use a digital camera to record the pattern of blue as you work to see if anything inconsistent happens. After a few hours your eyes go GA GA and you forget what pattern the blue was when you started, clean off and re-blue.
HSS tools can have the edge resharpened with a fine diamond stone, and on a flat tool remember you have two edges, so flip it over if things seem to slow.
Biggest Tip : - Start on a scrap part...
Have fun.
p.s
Should also add, don't worry about a messy looking scraped finish or aiming for total contact. To start with you are looking to optimise contact of you parts rather than making it look pretty. Regular contact spots across the surface is better than large gaps and remember how much material you are taking off. If the parts rock you need more than scraping.