There's a pretty simple procedure to do that :-
1) Find the hyperlink in edit html of your post.
2) Now add target="_blank" attribute in the 'Edit Html' mode just after the
href="URL of the story" in Blogger editor.
3) Now click on "Publish Post".
Eg:-
original : a href="URL"
modified : a href="URL " target="_blank"
You can also use the same logic for images aswell.