It is imperative that you Back-Up (Save a Copy of) your Blog Template - BEFORE and AFTER you 'tweak' it. 

One idea is to create a folder on your computer, say in My Documents file, called "Blog Templates - Back-Up" (or some such name).

If you have 'X' amount of Blogs, make 'X' amount of folders - one for each Blog.

Before, and After, working on the template, download a copy of your template to the relevant folder. It takes seconds to do. Easy.

To do so, takes a click Layouts - 'Edit html'. Then, select  "Download Full Template" in the "Backup / Restore Template" section of the 'Edit html' option. You then 'save a copy' to the relevant folder.

Then, if anything goes wrong, you are not completely ruined. You have a copy.