The main blog page file is the index.php which you would upload into the child theme. If you then open the /page-templates/ folder in the theme, you will find page templates, one in particular is:
In that you will see all the sidebars being loaded with code like this one:
IF this one you want in your blog, you would paste that in the index.php file where it is in the page template….right after the opening div:
Once done, you can try it out and put a text widget there with your banner or some sample text to see where it shows up in your blog main home page (where the posts show up). If you need this banner on the full post (single) view, then you have to do the same by copying the single.php file as well.