People have been enjoying this classic sandwich for years, but were not always calling it a BLT.
The BLT is an icon in the sandwich world. Even before the more recent trend of bacon mania, the BLT was always a popular choice with mass appeal. There is just something about the simple combination of crispy, salty bacon, juicy tomatoes, creamy mayonnaise, and fresh lettuce on toasted bread that creates sandwich perfection. The BLT is the perfect choice for lunch in Denver.
Bacon sandwiches have been around for a very long time, but they haven’t always been called a BLT. In 1903 the Good Housekeeping Everyday Cook Book contained a recipe for a club sandwich that included bacon, lettuce, tomato and turkey. The 1920 cookbook Calendar of Sandwiches & Beverages by Elizabeth O. Hiller also had a recipe for a bacon sandwich with tomato, lettuce and mayonnaise on toasted bread.
The term BLT doesn’t seem to appear in print until around 1941 and while the popularity of the sandwich grew in the 1950’s, the use of the term was not widespread. In 1958 Hellman’s mayonnaise advertised that their product was “traditional on bacon, lettuce, and tomato sandwiches.” It is generally believed that the acronym came from diner and restaurant workers, who developed their own lingo to communicate and used BLT as a shortened version of the name. By the 1970’s, however, the term appeared to be universally used.
Source: http://www.foodtimeline.org/foodsandwiches.html#blt