Write your back-cover blurb in present tense and in third person with active, direct language.
Quickly summarize the basic plot, but don't give too much away. Your blurb should indicate what your story is about in general and the central conflict, but avoid long explanations and sub-plots.
Introduce main characters by name and give a sense of their personality and motivation. Since plot often moves on the basis of a character's choices, a reader must know the character's basic motives and any specific elements of their story that help to explain or clarify the plot.
Indicate the setting of the story in vibrant, interesting language. Let the reader know if your story is set in a very specific place (such as a historical castle or famous location) and use descriptive language to make your setting interesting to the reader.
Leave the reader with some sense of mystery. Often, using questions can achieve this. For example, ending a blurb with "Lisa thinks she has found 'the one', but can she overcome her past and learn to trust again?" leaves the reader with a question that also provides a glimpse of the story's main plot point.