
Google allows your blogs to be served over an encrypted connection. This feature was rolled out long back for blogspot blogs. In order to make use of this feature, you have to do almost nothing, but this post will provide some more details around the same including setting up of http to https redirects.
If you are using a blogspot blog(i.e not a custom domain, then your blog will already be having an https address.
Blogger Custom domains are more like a Vanity URL for your BlogSpot blog. It will allow you to create a better and easy to remember URL. (for example : from yourblog1234.blogspot.com to yourblog.com)
Google’s domain registration service