How can I style the Comment Section of a blog post

Open your CSS editor by going to Appearance => Customize => Additionall CSS

Next, add the CSS there and save it. The live editor will show you results in real-time.

.comment {
    background: none;
}

Use any of the either. The first one is better. It will cope with the color of your blog-content if you change the blog-content background.

.comment {
   background: #111111;
}