I was curious about the history of the B.L.T. so I looked it up. Here is what I found.
The BLT evolved from the tea sandwiches served before 1900 at a similar time to the club sandwich, although it is unclear when the name BLT became the norm. It is currently one of the most popular sandwiches both in the United States and the United Kingdom, enjoyed by all social classes. The sandwich's popularity has led to a number of oversized reproductions (the current record for the "world's largest BLT" is over 209 ft/64 m) and a pop art sculpture by Claes Oldenburg.
Although the ingredients of the BLT have existed for many years, there is little evidence of BLT sandwich recipes prior to 1900. In the 1903 Good Housekeeping Everyday Cook Book, a recipe for a club sandwich included bacon, lettuce, tomato, mayonnaise and a slice of turkey sandwiched between two slices of bread. Whilst the 1929 book Seven Hundred Sandwiches does include a section on bacon sandwiches, the recipes often include pickles and none contain tomato.
The BLT became popular after World War II because of the rapid expansion of supermarkets, which allowed ingredients to be available year-round. The initials, representing "bacon, lettuce, tomato", likely began in the American restaurant industry as shorthand for the sandwich, but it is unclear when this transferred to the public consciousness.