Unfortunately, Squarespace does not have many templates which have the ability for a traditional, side-column layout. Side columns are especially useful to have an email capture system, or highlight categories or add a search bar so people can easily find multiple blog posts.
The FIVE template from Squarespace is the most versatile and useful for bloggers. Here is the link to the Five template: https://five-demo.squarespace.com/
This website uses the Five template, and you can see the side bar on the right hand side if you are on a desktop. For mobile devices, the navigation bar shows up down the page.
Five is really the only decent template for bloggers on Squarespace. Nothing else as a fully customizeable sidebar, so it's pretty much your only option if you want a traditional blog layout.