I'd choose neither until knowing more about them. This does not prove I am close-minded because your mother's argument is based on a fallacy called affirming the consequent.
She's saying that:
If you are prejudiced against both women and blacks,
Then you will not decide who to vote for.
Therefore, if you do not decide who to vote for,
Then you are prejudiced against both women and blacks.
The short form for this is:
If A then B.
B.
Therefore, A.
This fallacy fails to take into account that there could be multiple causes of B, not necessarily just A. Meaning that there could be other reasons to vote for neither candidate besides being prejudiced against both of them; specifically that we need more information about their political stances.
You could actually argue that choosing either of the candidates with this limited information shows closed-mindedness because you're demonstrating a preference for/against one of the candidates' attributes without actually knowing anything about them.
For more information about this fallacy, go
here.