Google rolls out a new feature that enables all blogspot domains to be on https encryption. It was first introduced for blogspot blogs last september 2015.
HTTPS is fundamental to internet security; it protects the integrity and confidentiality of data sent between websites and visitors' browsers.- Official Blogger Blog
This feature only applies for blogspot blogs under .blogspot.com domain. Https feature isn't supported for custom domain at the moment.
By default, blogger offers both options to use http or https for your blog as most of custom theme's doesn't work well or cause bugs with https encryption , so better run a test first before moving with https for your .blogspot.com domain.
You can enable this feature by going to Settings > Https > Select your Http Redirect option. Once published your blogspot domain will show green secure lock which means the connection is secure.
