You cannot set your entire wordpress.COM blog up to have a default setting so every link opens in a new window.
In any event, it's not avisable to do despite the "advice" that's being spieled throughout the blogosphere. It interferes with the ability of the reader to make their own choice and it can be extremely annoying to readers to have many open windows. If they are on a slow connection and it can cause crashing. I multitask and have many open windows so when I experience a single crash due to this removal of my ability to choose that's it for me. I click out of the blog and never return to it again.
How to set links to open in a new window
Dashboard > Links > Edit / Add new >
Advanced settings > Target (check the blank option)
Posts and Pages (using the link button to create them)
Select “Open link in a new window” from the Target dropdown. To correct links already inserted in posts and pages, switch from the Visual editor to the HTML editor and change this:
You upload the image, click File URL, insert it, then click on the image (in the visual editor), click on the edit button (mountain icon), click “Advanced Settings”, scroll down to “Target”, tick “Open link in a new window”, click Update, click Update Post.
Or, you upload the image, click File URL, insert it, then in the html editor add this to the image code (after href… etc… jpg, with a space before it):