The ECM calculates the fuel injector pulse width on the assumption that it knows all about your engine... displacement, injector size, mass air fow, etc. It uses an equation that includes the terms:
..... x (BLM/128) x (INT/128).......
The BLM is the long term fuel correction. This is what is stored in the ECM in volatile memory. The INT is the short term fuel correction, and its not "stored". The ECM recalculates the short term correction for every cycle of the calculation - about 9 times per second. It is intentionally forcing the closed loop A/F ratio to swing back and forth between very slightly rich (14.65:1) and very slightly lean (14.75:1). It has to do that for the cats to work.... when its lean the cat stores the excess oxygen, when its rich the stored oxygen combines with the unburned hydrocarbons (HC) and carbon monoxide (CO).
If the ECM finds that the long term average of the INT's is adding fuel (over 128), it ratchets up the BLM's to try and get the average of the INT's back to 128. It stores those values in the volatile memory..... that's most of what the ECM "learns". If the BLM's are above 128, the ECM is responding to what it BELIEVES is a lean condition. IF the BLM's are below 128, its responding to what it BELIEVES is a rich condition.
The BLM's are developed during part throttle/part load conditions, when the ECM is operating in closed loop (O2's are being used to modify the fuel rate). They are not learned when you are running at WOT, so they are not an indication that the engine is leaning out at peak power output. They are used at WOT to calculate the fuel rate (but only if they are above 128). A BLM above 128 indicates that something in the fuel system is not responding the way the ECM thought it would.
For example, if your fuel pressure is too low, the injectors don't flow the amount of fuel the ECM is expecting, it runs lean, and the O2 feedback elevates the BLM's above 128. If you had a vacuum leak, same result. If you have an MAF sensor and its dirty and reporting less mass air flow than is actually entering the engine, its a lean condition. If you have a speed-density setup, and have modified the intake and exhaust to improve the engine's breathing, but have not had the programming modified with changes to the volumetric efficiency (VE) tables, it would run lean. These are all "TRUE" lean conditions.
But sometimes, the O2 sensors give the incorrect feedback, indicating a "FALSE" lean condition. The main examples of this would be 1) faulty O2 sensor(s), 2) misfires, 3) exhaust leaks before the O2 sensors. All of those trick the ECM into thinking the engine is running lean, and it adds a bunch of extra fuel, using the BLM's, that the engine does not need.
I have this online writeup that includes the A/F management aspects of the LT1 engine management system, but its pretty much the same for the 3rd Gen fuel injection systems: