Caroline Kaiser has some thoughtful suggestions about how to write a back cover blurb. Go read her advice on her blog.
It can take weeks, she says, and that many iterations too, to get a blurb you are happy with. Jotting down a version now — whatever stage you are at in the writing process — can lead you to a better blurb in the end and help form and solidify the book you are writing.
Jot down a blurb now, and file it away like Caroline suggests.