Born in 1782 in a cabin west of Annesburg, Josiah's story seems to be mostly of legend. This is apparent when it is claimed that, at the age of only 3, he was given a rifle and was able to kill almost any animal, as well as being able to fight a bear into submission. It is said that after these "feats" he achieved that he went to traditional schooling, only to leave after three days.
According to the story shown in the Valentine movie tent, he ran away from home and headed to the unsettled west. There, he allegedly killed all the bison and Native Americans in the area, and met a 12 year old Native girl who he pitied on. He subsequently decided to marry her, and the two had children together. However, Josiah Blackwater's wife and children soon fell prey to Typhus.
At some point later, it is said that he killed a mountain lion to protect an opossum that he would befriend and eventfully wear as a hat. At a later point in his life, he founded the town of Blackwater on Flat Iron Lake. Not too long afterwards, he met Ruthenia Hellsby at a barn dance, got re-married and had a total 14 children. During this time, he reportedly had a cattle empire before eventually moving back east to Saint Denis, where it is said he mostly indulged in prostitutes and gluttony after claiming the people of his own town can "go to hell, and [he] will go to Saint Denis".
After his pleasurable time in the slums of Saint Denis, he got the idea to run for Congress. He, miraculously, became a congressman, supposedly wearing his opossum hat everyday as well as carrying his gun while exclaiming "I may live in barbaric splendor, but I always remember... keep your friends close and your gun closer... 'cause you don't know when you might have to shoot 'em". While his story is possibly entirely fabricated, this is all that is known about him.
- In the Wheeler, Rawson & Co. Catalogue in Red Dead Online, it is said that Josiah Blackwater rode a "Kentucky Mountain Saddle Horse", possibly alluding to the in-game Kentucky Saddler.
- Josiah Blackwater is most likely based on many real-life pioneers of the American frontier, such as Davy Crockett, James Bowie and Daniel Boone. He shares many similarities with these, most notably in terms of making way for settlers and wearing hats created from animal hide.
- Despite the movie that can be seen in game giving his birth year as 1787, a banner seen in Blackwater during the epilogue in 1907 states the town is celebrating the 140th anniversary of its founding during that year.