Back in September 2015 Google added a setting in Blogger’s back end that allowed users to turn on HTTPS for their blogs. Now another option is now in blogger back end which is “ HTTPS Redirect.”
if turned on, it will automatically redirect all blogs visitors to the HTTPS URL instead of the old HTTP link. This redirect works automatically, any users accessing older links ( HTTP ) embedded in various sites,forum or social networks will land on the blog’s HTTPS version. This could cause an error and it will only affect
subdomain blogs I.e BlogName.. com not the custom domain name BlogName.com. Google as created a tool spots mixed content errors. Every blog owners with custom domain are advised to correct URLs and fix their sites immediately.

Source: www.geeklogic.net/https-now-on-default-for-all-blogger-sites-/