Your blog depends upon traffic for its success - and similar to custom domain migration, will suffer a period of lower search related traffic, while being upgraded to HTTPS. Larger blogs get better search reputation, all other details being equal. While your blog is being reindexed, after you enable "HTTPS Redirect", you will have a blog with two different base URLs - and both URLs will reference two smaller blogs, and have lower search reputation. All posts in your blog will not be reindexed under an HTTPS URL, immediately. Some posts will remain indexed under the HTTP URL, as others are re indexed under the HTTPS URL, one by one. While being reindexed, your blog will look like two smaller blogs - and both URLs will suffer from lowered search reputation.
What Blogger won't tell you