I guess it may depend on what the mark on the card was actually originated from. If a TPG determines that it is indeed some sort of mark, they would dock you for that and if you continue to experiment until the mark is gone, the erasure would be obvious at that point and the card would show the alteration. So my vote is that you were probably damned either way
Regardless it is a nice card and you should enjoy it as such. And honestly, those types of cards are the best 1.5's to have because they are absolutely beautiful with no "real" flaw to take away from the card's appeal IMO.