I really don't know what is the meaning of "blue print"

I think u solve this problem yourself.

The blueprint process was developed by the British astronomer Sir John Herschel in 1840. The photosensitive compound, a solution of ammonium ferric citrate and potassium ferricyanide, is coated onto paper. Areas of the compound exposed to strong light are converted to insoluble blue ferric ferrocyanide, or Prussian blue. The soluble chemicals are washed off with water leaving a light-stable print. blueprint is a plan or technical drawing usually documenting an architecture or an engineering design. More generally, the term "blueprint" has come to be used to refer to any detailed plan.