Fast blink is usually a shorted cell, excessive current draw, or cell resistance out of range during initial test. There were multiple versions of the older "square block" charger. Which one do you have?
There was the 3053564 where the output was 12v @ 500ma, which if I recall, was for the NiMH style packs, not Li-ion.
There was the 3053568 with an output of 7.4v @ 800ma, that was for the early Li-ion packs
And the 3053572 which outputs 8.4v @ 1300ma, which is the current charger
The first charger would destroy the pack - wrong voltage. The second would charge, but probably never top off as the voltage is too low (a full 2 cell Li-ion grip is around 8.4v). The last charger is the current H5-H6 charger, but obviously designed for the 2900 batteries as the charge rate (the "C") is typically 1/2 of the cell rating so it was created around the 2600-2900 era. It'll still charge the 3200 battery to full as the voltage is correct, but would take an extra hour or two. The "7.2v" rating for the pack is a nominal rating, as the charge is a range from full to dead.
It sounds like you have the second charger (I hope it's not the first). Measuring one of my 3500 packs when I first upgraded them showed the following:
100% charge - 8.35v
75% charge - 7.90v
50% charge - 7.25v
25% charge - 7.05v
10% charge ("dead") - 6.65v
These measurements were with a brand new, first cycle pack. After some usage, they'll change a bit, but I haven't had time to re-measure the packs as it takes forever to do so. I just wanted a reference for future testing of the battery should something weird occur. If there was some wear on your pack, that 75% charge with the older Li-ion charger would probably be correct.