A few days ago, I wrote a post about why the word barrel is abbreviated bbl. I thought I had done my homework. I had checked my sources and it seemed like my post was reliably sourced (going back as early as 1904) when I had attributed the abbreviation bbl to the use of blue colored barrels in the Pennsylvania oil industry in the 1860s.
Thankfully, an anonymous reader wrote in with a tip saying that bbl was in use long before the Pennsylvania oil industry. Sure enough, I did a little poking around on the internet and found some old cargo manifests from the early days of sail. I found one as early as 1764 that use the bbl abbreviation and it was certainly in use even before this.
(Manifest of the brig Sally, dated September 11, 1764 - note the bbl abbreviation)
So where did bbl come from? I contacted three reference librarians across the country, and after doing their research, all three came back with the "blue barrel" answer. When after I explained my earlier cargo manifest finds, one librarian from New York still put up a defense saying that "blue barrel" was well researched and had a long history of documented evidence.
One site I found took a couple of good guesses stating that the "b" may have been doubled originally to indicate the plural (1 bl, 2 bbl), or possibly it was doubled to eliminate any confusion with bl as a symbol for the bale. Of course these are just guesses.
I'll keep looking for a satisfactory answer and I'll let you know if I find anything. Of course if there's someone else with more information, feel free to post in the comments.