In order to configure blog design in Magento 2 Blog navigate to Stores > Configuration > Magefan Extensions > Blog and find the Design section. Note that this option is available only for Blog Plus and Blog Extra users.
In this section you are able to choose the Layouts for Blog Post, Category, Tag, Author, Search and Index Pages. It can be empty, 1 column, 2 columns with right bar or left bar or 3 columns. In case you have some designs on your Magento instance they will be displayed here as well.