Back in September, Google announced that HTTPS support was coming to, 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 errors often stemming from your template, gadgets, or blog post content.

Those who get Mixed content error can;

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!