Actually Blueprinting refers to the original factory blueprints for measuerments rather than using any tolerances. When the engines were designed, an original set of blueprints were developed for them and these are what manufacturing companies used as a baseline for manufacturing engines but because no two castings are exactly the same ( near all engines of any sort start with a casting ) they introduced tolerances to allow non a slight amount of non conformance to the original design.
Example: Chevvy smallblock originally called for a deck height (crank center line to top of cylinder) of 9 1/2" , but most I've measured come out over and up to 9 3/4" . To properly blueprint that block, I would have to mill the deck height parelell to the crank at 9 1/2" because that is what the original blueprints called for. This process leads to performance because carefully measuring and machining these dimensions makes sure that all the cylinders are exactly the same as what the original designer wanted with all the variences taken out.