How to fix black 'veining' around drain hole of vanity sink?

I don't know what this is made of; it is part of a bathroom vanity, where the sink and vanity top are one piece. It is 11 years old and a few years ago these black 'veins' started showing up. They are nowhere else on the "cultured marble" or whatever it is, just around the drain. I have scrubbed with baking soda and SoftScrub with no change at all. Any ideas? TY all.

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