Unfortunately, there is no such thing as a genuinely black strawberry. Images you might see on the internet of pure black strawberries are either photoshopped or painted. In fact, black strawberries started out as art.
Now that you know that black strawberries do not exist, you should be able to identify any listing for black strawberries or their seeds as a scam.
While black strawberries are a myth, strawberries come in more colors besides just red. Though rarer, you can find yellow/golden, white, or purple strawberries.
The Alpine Yellow Wonder Strawberry is one of the most visually stunning strawberry varieties. They are a slightly subtler yellow than a banana or a lemon and may even be described as golden sometimes.