Should I purchase an SSL separately?

You don’t need to buy.
Superblog gives a free SSL (generated by Let’s Encrypt) for your blog.
If you host your superblog on a subdomain (blog.example.com), then you can utilize the free SSL.

If you host superblog on subdirectory (example.com/blog), then the SSL present on your main domain will be sufficient. Superblog can’t generate an SSL for your main domain.