I know Elizabeth said that every gift is both light and dark, but I don't think they were just talking about the powers they have exactly. Going by the fact she was talking to Mirabella I feel as if she may have been trying to state the fact of the queens being both light and dark. She didn't know Mirabella well at this time, so she didn't know how Mirabella felt about her sisters. She could have been hinting the fact without openly saying it. We've seen a dark side and light side to each queen, as life and death situations bring out sides of a person they never seen. It may have been a simple quote that wasn't as deep as I'm making it but I feel as if there was a deeper meaning.