Finally, now blogger(blogspot) support SSL certificate(HTTPS) on custom domain. You can get free SSL certificate for your blog. Here a official trick for enable 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 than it already activated SSL by default.
How to enable HTTPS on Blogger
1) First you need to login to blogger.com for go on dashboard
2) After that you need to go on settings.
3) Now click on simple.
4) Look for HTTPS Availability and HTTPS Redirect option. select YES in both field.
Now your free SSL certificate will be activated. It may take some time for activate.