Originally, a "blueprint" would result when an architect, engineer, or draftsman drew a plan in pencil or ink on thin paper ("velum"), then laid that paper over a sheet of special chemically-treated paper and exposed it to a strong light for several minutes. Upon treatment with ammonia fumes, the chemically-treated paper would turn blue where the light struck it and remain white where the pencil/ink lines blocked the light (ie, it produced a "negative" image). This permitted quickly making accurate reproductions of the original drawing.
Over the years the process was changed such that the image was no longer "negative", and eventually it was replaced with xerographic reproduction, but the term "blueprint" stuck.
Since a blueprint was what, eg, a carpenter referenced when building a house, the term "blueprint" came to mean simply "plan", and it is often used in a metaphorical sense when there is no actual drawing per se. (Eg, "The congressmen conferenced and produced a blueprint for the new tariff law.")