It will fade, with time. You could try vitamin E or BioOil to see if you can hurry the fading along.
Would it be rude to completely ignore the question from a perfect stranger? Continue on with whatever interaction you were having as if they didn't ask the question. So if you are checking out in the grocery line and the cashier, during the process, says, 'Where did you get that scar?', you could answer, 'I'll be using my credit card, today, thanks.' or 'I brought my own bags.'
It might also have the secondary effect of giving the person a clue that they'd just asked something extremely personal and horribly rude.