Story 1: Sweet Oil of Vitriol

The crime mystery of the story is loosely based on the book Kill-Devil and Water by Andrew Pepper, set in pre-Victorian London. In order to make the story into a playable mystery a connective tissue was neccessary to underrun all relevant parties upon the island. This was achieved through the addition of the anaesthetic that would allow for the murder to be performed within the Galley, and the blinding sickness, that would offer several plot connections and a solid basis for the composition of the drug. The story behind those two things is the following:

Sweet Oil of Vitriol

Sweet Oil of Vitriol (today known as Diethyl Ether - (C2H5)2O) is an anaesthetic, colourless, organic compound with a characteristic smell.

It was chosen as the drug of my story mainly for two reasons:

1. Its relatively easy composition. Its first officially recorded composition was done in 1540 by Valerius Cordus, though it might have been known and composed since 1275.
2. The fact that the ingredients for its composition would be readily available in a volcanic island of the Caribbean. Sulfur could almost certainly be found in any volcanic area (and even if it wasn’t, sulfur would have been imported by the banana cultivators as an insecticide. Saltpeter could have been harvested by nitrous beds formed in the lower layers of bird poop upon heavily encrusted rocks (In fact many real islands, like Nauru, survive even to this day due to saltpeter production from bird poop). If sulfur and saltpeter are burned together they produce sulfuric acid (Oil of Vitriol), in a process known and recorded since the 17th century. Producing ethanol would be the next step. Luckily bananas produce great amounts of ethanol during their ripening process (more than any other fruit). Distilling a mixture of ethanol and sulfuric acid produces a very good amount of Diethyl Ether.

The idea to use the ether as a drug to alleviate pain and possibly cure a disease also stems from historical foundations. The analgesic properties of the ether were first observed in chickens by Paracelsus in the 16th century. By the 19th century the drug was widely sold as a cure all, due to its power to alleviate symptoms, while it was also used recreationally, especially by women, for whom alcohol drinking was considered indecent. This fame of the drug , that explains its elevated price in my story, is a certain anachronism in historical terms.
The obscure nature of its anaesthetic properties in the story is historically grounded on the fact that the drug was not used as an anaesthetic before the 19th century, with its first public demonstration in this role being performed on  October 16, 1846 by William T.G. Morton.
The fact that Diethyl Ether is an extremely flammable (F+) fluid was a sweet bonus. It is a category IA flammable substance with a flash point of -45^oC and an autoignition temperature of 160^oC.
(In case you have  a lot of free time and are not scared by disclaimers, you can easily make Diethyl Ether)

The Eastern Blindness

The Eastern Blindness was based upon the Trachoma eye infection, an infection caused by the bacterium Chlamydia Trachomatis, the genus of bacteria responsible for genital Chlamydia in humans.
Trachoma was known since ancient times and was in fact the leading cause of infectious blindness in humans until quite recently.
Trachoma eye infection is actually not transmitted sexually, belonging to a different strain of the bacterium. It is transmitted through direct contact with eye, nose, and throat secretions from affected individuals, or contact with towels or washcloths, that have had similar contact with these secretions. Flies can also be a route of mechanical transmission. Despite this, its connection with genital chlamydia was established quite early and the two infections were often attributed to the same (sexual) causes.

Untreated, repeated trachoma infections result in entropion—a painful form of permanent blindness when the eyelids turn inward, causing the eyelashes to scratch the cornea.