Hmmm... I think it can be explained all together:
Blacklisted means that:
That is, after a given update in the DSi system, the card was blocked forever.
Not all cards are blacklisted, though, so:
that is totally normal, as probably his card is newer and not in the blacklist.
It depends on what is your point of view, the card is not defective, but it was blacklisted in the DSi system, so it will not work, in the other hand other cards will, but this is not a defect of your card, it is a limitation.
The card is not defective, and you could make it work, but for this
The only other option is to get a newer card that is not blacklisted.