Board Thread:Questions and Answers/@comment-5876739-20160929075540/@comment-27795079-20161004082350

Technically they've never actually established this on the show, it's just assumption on our part for the sake of articles like House Blackfyre. As I said earlier, "common practice" doesn't mean we can apply it to all bastards. For example, we can't use a black ironwood tree sigil for Elsera and Josera Snow because they never use one in the game... they live as wildlings beyond the Wall, so likely don't have any sort of heraldry at all.