Does she know her colors yet? I would be very surprised if she could. I also would be very surprised if she cared.
Parents tend to worry about children's color preferences. Boys who like pink. Girls who like black. Heaven help the mother of a child who insists on only wearing white - unless dad does the laundry. Then, heaven help him
Colors are a much bigger deal for parents than kids. The odds aren't that your daughter is colorblind. The odds are very far from the chance that your daughter will be a goth. Let the sweet little child play without any cares on your part.
She may just find the high contrast between black and white attractive (I assume she's coloring on white paper? If you gave her black paper, she might pick up a white pastel to draw on it. Who knows?
If you're worried, she should have a doctor visit coming up. Mention it to the doctor. He might have her tested for her vision. They test for color blindness later.
Meanwhile, it's cool that she has such well developed preferences. Celebrate her individuality! Black backgrounds are wonderful for pops of color at a party.
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