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It depends on what you have in mind… If you just want to remove the black strip you can create a child theme, then add this to the stylesheet (style.css):

.post-format-content {
background: none;
}

Do note that there is no image below the strip. It’s my guess you will want to change the thumbnail size also. You probably can do so in media settings (/wp-admin/options-media.php), but you may need to regenerate the thumbs. For that you can use a plugin like Force Regenerate Thumbnails.

GL!