But if low voltages and amps can cause these oxides to form, surely the blade would have been rejected after being subjected to high volts, high amps in a lightning strike?
If such currents cause havoc with the titanium matrix, then I would expect the blade to have been scrapped. So either the damage, IF it occurred (as it's not clear from the AAIB findings just yet) was not apparent, or the check was not performed correctly, if I understand your statements correctly.
And as other operators have recently removed blades that have suffered lightning strikes in the past as a precautionary move, there must have been quite a number over the years that have been checked, found satisfactory and returned to service.
I cannot believe that if lightning had such damaging effects these blades would have been allowed back on aircraft. But then again, I may be proven wrong..