When to use target='_blank' :
The HTML version (Some devices not support it):
This link will open in new window/tab
The JavaScript version for all Devices :
The use of rel="external" is perfectly valid
and for Jquery can try with the below one:
$("#content a[href^='http://']").attr("target","_blank");
If browser setting don't allow you to open in new windows :
href = "google.com";
onclick="window.open (this.href, ''); return false";