Finally, now blogger(Blogspot) support SSL certificate(HTTPS) on a custom domain. You can get a free SSL certificate for your blog. Here is an official trick for enabling HTTPS on your custom domain Blogspot blog.
Before you start, make sure you have a custom domain blog because if you have a subdomain(xyz.blogspot.com) blog, then it already activated SSL by default.
How to enable HTTPS on Blogger
1) First you need to login to blogger.com for going on the dashboard
2) After that you need to go to settings.
3) Now click on simple.
4) Look for HTTPS Availability and HTTPS Redirect option. select YES in both fields.
Now your free SSL certificate will be activated. It may take some time to activate.