Yes, I agree that for centuries, black has been much worn by Mediterranean women who lost basically any family member, and for the rest of their lives. However, I don't think Rilke is picturing his sibyl in the Christian era to begin with. The term sibyl is heavily freighted with Classical weight, is about the best argument I can provide. Rilke would have to do violence to that tradition to resituate her. OTOH, his conception of what qualifies as ausgebrannt where skin is concerned might not correlate entirely with our own. That is my little point. I don't think it likely that ausgebrannt refers here to clothing.
Cheers, good discussion,