Even if someone took that article seriously, which they shouldn't since about.com isn't an actual history or even scholarly source(neither is the writer of said article), all that says is that it's inconclusive of her having any black relatives. What is confirmed is her white ancestry with many scholars agreeing, and since a question mark, even for up to half of her heritage, doesn't equal confirmation in any direction then you should go with what is already confirmed about her. Therefore, it makes sense to cast her as a white actress. If there is confirmation to the contrary then of course it should be represented. Until then, it's best to stick to the facts at hand.