Cyanotypes---simple, blue, and brilliant. While considered photography, they fall into the bucket of "alternative photography" which includes other experimental type imaging processes such as double exposures, cross processing, and pinhole, to name a few.
The process was invented by Sir John Herschel in 1842 as a means of reproduction---specifically notes, diagrams, and other low-thrill items to support him in his scientific and astronomical pursuits. This process allowed him to make blue prints or copies of his work...blue prints...BLUEPRINTS. Yes, this is how blueprints were born. You're welcome Jay-Z.
Below is the work of Anna Atkins, botanist and photographer who produced many works, the most famous of which, Photographs of British Algae: Cyanotype Impressions, was also the first book EVER to be illustrated with photographic images. The cyanotype below is a sample from this publishing.
While other, simpler techniques are available in the photography world, cyanotypes are still alive and well. There is something captivating about the uncertainty and fragile nature of what is being captured through the process. In the short time I've been familiarizing myself with cyanotypes, I've been impressed with the process. I'll be looking to get my hands on some ferric ammonium citrate and potassium ferricyanide so I can relive the glory days of my short-lived career as a chemist and try to make some bright blue art.