A BLAT is a BLT with one ingredient added, and the addition changes the sandwich more than its single extra letter suggests. Avocado does not sit alongside the bacon, lettuce, and tomato as a fourth equal part; it rebalances the whole thing. The classic BLT runs on tension: hot crisp bacon against cold sharp tomato on toast that is fighting to stay crisp. Mashed or sliced avocado introduces a soft, fatty, low-acid third texture that softens that tension into something rounder and more filling. Whether that is an improvement is a matter of taste, but it is unmistakably a different sandwich, and the avocado is the variable that makes it one.
The craft is the same moisture problem as the BLT, slightly relaxed and slightly complicated. The tomato still weeps and is still salted, drained, or held off the toast by a barrier. Avocado is useful here as part of that barrier: spread against the crumb it waterproofs the toast much as mayonnaise does, while adding richness rather than tang, which is why a BLAT often needs less or no mayonnaise. It also has to be ripe and used at once, because under-ripe avocado is hard and bland and browned avocado is grey and bitter, so this is a sandwich with a narrow ingredient window the BLT does not have. The bacon is still cooked properly crisp to hold its texture against the soft fruit, and the toast is still built and eaten quickly before it surrenders.
The BLAT is itself a codified mutation of the BLT, and the family keeps going from there. The plain BLT is the parent without the fat; adding chicken or turkey pushes it toward a club; the toasted three-decker braces the same components with a middle slice. Those deserve their own articles rather than being crowded in here.