HTTPS For custom domain is made available for the users by Google. You just have to enable the HTTPS and your blog will have the HTTPS:
You don't need to purchase SSL certificate. When you select yes all setup will be done by the google to provided HTTPS on your blog
|HTTPS for a custom domain on Google blogger|
It will good to redirect all your secured and non-secured traffic to the secured site. So Select yes on HTTPS redirect
|Redirect all traffic to the secured version of your blog|
If you are using google blogger and haven't done it yet do it.