Serving a secure version (HTTPS) of your blogger.com custom domain was possible only by using Cloudflare services where is possible to enable Flexible SSL for the domain an gain the benefits of HTTP over SSL encrypted traffic to your site, including advanced SSL configurations like "Always on SSL", HSTS and such.
Flexible SSL from Cloudflare does not create an entire End to End HTTPS, meaning the connection to your blog will be encrypted only between client computer and the first Cloudflare node (server) and requests made by Cloudflare to blogger.com custom domain are done on a plain HTTP connection. Using Full SSL / Full(strict) SSL as a complete End to End HTTPS was not possible for a blogger.com custom domain.
Even if Flexible SSL is enough for a blogger.com custom domain, there are at least two problems with this configuration, first, all URLs from the blogger generated sitemap.xml have http protocol instead of https, second, for mobile visitiors there is one more roundtrip that need to be done because blogger.com redirects mobile visitors to http version of the page and the Cloudflare redirect it again to the https version.
Time has changed and now blogger.com offer HTTPS for custom domains. Now we can full benefit of the HTTPS for our blogger.com custom domain, including https URLs in the sitemap.xml and and a better loading speed for mobile visitors. All what is need are only three settings that must be done.
Cloudflare Crypto module
Here, on the SSL settings we need to switch from Flexible SSL to Full or better to Full (strict) SSL mode. From this point, until we finish, few minutes later, our blog will not be available, instead, a Cloudflare page informing that Cloudflare could not establish a SSL connection to our site, will be displayed.More informations about Cloudflare E2E HTTPS can be found here: https://support.cloudflare.com/hc/en-us/articles/200170416-End-to-end-HTTPS-with-Cloudflare-Part-3-SSL-options
blogger.com, blog basic settings
On blogger.com, blog admin, settings, basic, HTTPS, we need to change HTTPS Availability from No to Yes. blogger.com will inform us that will take some time so we need to be patient, just refresh the page from time to time until we see that HTTPS in enabled.
After a while, when HTTPS from blogger.com is enabled, we need to disable HTTPS Redirect (switch to "No") as it will interfere with Cloudflare HTTPS redirections.
That's it, now traffic to our blogger.com custom domain is Full Encrypted, we have https links in our sitemap.xml and no extra redirects for mobile visitors.