The blue light, coming on quickly, only means the voltage on the battery under charge went above about 13.3 volts (IIRC). One might assume that this means that the battery under charge (from alternator or outside charger) is in good shape and all is right with the world, but that would be just a WAG. It maybe good, or it may be that the battery under charge simply has a surface charge, that the internal resistance is high due to age and plate sulfation, and that the battery is on its last ragged shaky edge.
Voltage reading at rest is only one parameter that may "suggest" you have a good battery. It's a good check for sure, but doesn't always tell the whole story or even the truth. To test for battery health you would need to do a load test, or measure the battery's internal resistance. IMO a load test is best.
Note that you can do I quick, down and dirty, load test with a voltmeter (even the one on the panel may work if the display stays on while cranking (some don't). Watch the voltage as you crank the engine, ideally it should stay above 11 volts while cranking before the engine starts and the engine charging system raises the voltage.
I just replaced the main battery on my '21 SE (wrote about that in a previous post). I believe this original battery was defective on delivery to me. Probably due to all the part shortages and delivery delays, it was in stock for more than a year and was not charged every 6 months in order to keep it healthy (Pb batteries lose some 5% charge per month in storage, and plate sulfation starts in earnest as the battery charge drops below 50%). Upon removal I load tested the battery which showed bad. I had another, same size battery, on the bench that was at least 6 or 7 years old, but kept charged - so slapped it on the same tester. It showed "weak", but way better than my stock battery.
