Yes, blogging on Squarespace is free for individuals. However, if you want to create a blog for a business, you will need to purchase a plan. There are three different plans available, with different features and pricing: Starter, Standard, and Enterprise. The Starter plan is the cheapest and includes only limited features such as no stats or analytics, no private pages, and no custom domains.
The Standard plan includes all of the features of the Starter plan plus more advanced tools such as custom domains, stats and analytics, and a private blog. The Enterprise plan is the most expensive but offers the most features and flexibility, including the ability to have multiple blogs on one account, unlimited custom domains, and support for add-ons such as social media tools.