Back in September,we announced that HTTPS support was coming to .com,making it possible for you to encrypt connections to your blog; since then, many of you have enabled HTTPS for your blogs. In some cases, not all of your blog’s functionality has worked over HTTPS due to mixed content errorsoften stemming from your template, gadgets, or blog post content.
To help spot and fixsome of these errors, we're releasing a mixed content warning tool that alerts you to possible mixed content issues in your posts, and gives you the option to fix them automatically before saving.

Get full from the source:: http://www.gisttrace.com/2016/04/new-update-to-blogger-post-editor-to.html