The BLAT is a BLT with avocado, and the single added ingredient changes the engineering more than it changes the flavor. Bacon, lettuce, avocado, and tomato on toast with a layer of mayonnaise is the whole build, and the avocado is doing structural work the original never asked of anything: it is a soft, fatty layer that has to be placed so it does not turn the already-precarious sandwich to mush. The defining decision is where the avocado goes, not that it is there.
The craft is the BLT's craft with one more variable to manage. The bacon is still the flavor and the spine, rendered crisp and laid in a flat overlapping layer so every bite gets some and the soft sandwich has a little rigidity. The tomato is still the threat, a ripe slice shedding water the moment it is cut, salted to season it and walled off from the toast by mayonnaise spread edge to edge. The lettuce is still the cold crunch the bacon and tomato lack. The avocado is the new problem: sliced or mashed, it is rich and wet and contributes no structure, so it is placed against the lettuce rather than against the bread, using the lettuce as a barrier the way the original uses it to shield the bottom slice. Mashed, it can be spread thin as a second sealant; sliced, it has to be thin enough not to slide the stack apart on the bite. The toast still has to be crisp and recent, and the sandwich is still on a clock, because adding a high-moisture layer to a build that was already racing the tomato's water only shortens the window before the crumb gives.
The variations stay inside the same fragile frame, and each deserves its own article rather than being crowded in here. Add a fried egg and it leans toward a heartier breakfast build; run it on a long roll and it edges toward a club's territory; the club sandwich itself is the BLT's braced descendant, a third slice of toast solving the instability the BLAT, like the BLT, lives with on purpose. Each is a single move on a sandwich that barely has parts to move, which is exactly why it earned its own name.