In order to manage Custom HTML Blog in Magento 2 Blog sidebar, please go to the Admin Panel > Stores  > Configuration  > Magefan Extensions  > Blog > Sidebar and find Custom HTML section.

Custom HTML

1. Enable or Disable the Custom HTML.

2. Set the Sort Order to manage its position on the StorefrontThe higher the number of the position, the lower it will be displayed on the Sidebar.

3. Enter the HTML text you want to be displayed in the Sidebar. You can also include custom blocks/templates and widgets here.

Once you've finished don't forget to Save Config and Flush Magento Cache.

Magento 2 clog custom html configuration

 4. Check the blog Custom HTML out on the storefront.

Magento 2 blog custom html

There is the "Custom HTML 2" available for adding to the Magento 2 Blog Sidebar which is configured the same way.

Magento 2 blog custom html configuration