“This paper will prove valuable,” wrote John Herschel in a scientific memorandum on April 23, 1842, noting the effect of sunlight on a sample he had treated with “ferrocyanate of potash.” The light turned the chemical blue, leading Herschel to believe he had found a basis for the invention of color photography. He had not—nor would he live long enough to witness the true usefulness of his discovery.

A British astronomer and chemist, Herschel had already played a crucial role in the 1839 invention of the black-and-white salt print—the first photographic negative—by finding a way to fix, or set, the fugitive image with sodium thiosulfate. His obsessive search for other photosensitive chemicals led him to try out everything from vegetable extracts to dog urine, as well as the then new pharmaceutical known as ferrocyanate of potash, a substance now called potassium ferricyanide. The ferrocyanate produced a strong image, particularly when combined with another pharmaceutical called ammonio (ammonium ferric citrate), and the image proved permanent after washing. Herschel dubbed his invention the “cyanotype,” but he was deeply dissatisfied with it, because he could not coax the chemistry to produce a stable positive image—only a negative. Most photographers shared his opinion, shunning the strange cyan hue in favor of conventional black-and-white pictures.

Only in 1872, one year after Herschel died, was the cyanotype revived, when the Paris-based Marion and Company renamed his invention “ferro-prussiate paper” and began marketing it for the replication of architectural plans. (Previously, they had been copied by hand, which was expensive and prone to human error.) At the 1876 Philadelphia Centennial Exposition, the process reached American shores, where it finally met success as the blueprint, the first inexpensive means of duplicating documents. All that was required was a drawing traced on translucent paper. Pressed against a second sheet coated with Herschel’s chemical under glass, the drawing was exposed to sunlight, then washed in water. The blueprint paper recorded the drawing in reverse, black lines appearing white against a cyan background.

Occupying the top floors of office buildings where there was ample sunlight, blueprint shops thrived for nearly a century, only gradually phasing out Herschel’s chemistry for less labor-intensive processes such as the diazo print and the photocopy from the 1950s to the 1970s. Today most architectural plans are digitally rendered, and Herschel would have marveled at the color gamut of the modern laser printer. Yet he would have been puzzled, given his failed efforts to print in full color, to see that when we want to communicate an innovative new plan, we call it a blueprint and output it in cyan.