Back in September, Google announced that HTTPS support was coming to blogspot.com, making it possible for you to encrypt connections to your blog; since then, many of you have enabled HTTPS for your blogs.
Those who get Mixed content error can;
In some cases, not all of your blog’s functionality has worked over HTTPS due to mixed content errors often stemming from your template, gadgets, or blog post content.
Go to Template>>Edit HTML and from there press Ctrl+F (it will open a search bar) find for "http:" and it will highlight all the http words in your template. Now just change all "http:" to "https:".
Make sure all links you change to "https:" work fine by opening them in a new window.
I hope it helps!