Many authors note that writing the book blurb and slug are as difficult as writing the book itself. I certainly agree. These are extremely important, since they introduce and hopefully lure potential readers to your story. Give it all the time necessary to do the job.
It’s a good idea to have your book blurb in several different lengths. What you include on the back cover can be fairly long. However, when you post your book on certain sales or promotional sites, they may have a word limit, sometimes as short as 100 characters! Thus, it’s handy to have them already written so when you’re in the middle of posting your book somewhere you don’t have to stop and edit it down on the fly. Thus having one ready that is full, medium, and super-short in length will save time and frustration as well as assure top quality.