1. First log in to your Blogger account and back-up your template.
2. Then click Template >> Edit HTML
3. Chose Proceed to start editing the HTML code.
4. Click on "Expand Widget Templates"
5. Ctrl F and find the below code:
<head>6. Just replace above code with code given below
<head> <base target='_blank' />7. Now save your template and you are done.
Now check your blog links, all links in your blog will now open in new tabs.
Note: Some bloggers may prefer some links not to open in new window/tab but to open in the same window, add the attribute target='_self' for those links you want to open in the same window/tab.