As we create blog posts we usually link to external sites. We want those links to open in a new browser tab/window. 

There are multiple ways to do that. It is possible using WordPress's link to check a box saying open this "Open link in a new window/tab".
This, however, will take lots of time if you have lots of blog posts. Manual tasks usually can easily be automated.

By using the following WordPress plugin called: External Links you can make all of the external links to open in a new browser window/tab.

http://wordpress.org/extend/plugins/sem-external-links/