I have had a similar experience with 2 aloe vera bought from the same shop same day! They got the black spots a few weeks later and I cut the infected leaves off, repotted both with the same mix and put one outside! The one inside got nice new healthy leaves but the black spots came back whereas the one outside didn't! I think overwatering occurs indoors with these aloes when there is not enough air circulation and not enough sun! The one outside is always drenched with rain and the soil is always wet so I don't think it was overwatering that caused it! But it definitely contributes.
