Set default overlay color to black

[akasunil]

What?

Fixes #49785

Why?

Default overlay color isn't set in cover block, but we were loading black color though css, but the actual color picker shows nothing set.

How?

Set default value to black, It will show black color selected in overlay color picker.

Testing Instructions

1. Add cover block
2. Check overlay color in block setting

[github-actions]

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Co-authored-by: sunil25393 <sunil25393@git.wordpress.org>
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Co-authored-by: Olein-jp <olein@git.wordpress.org>

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