By now enough people know how to middle click, ctrl click, command click, context menu click etc to open links in new tabs, they don’t need your help and they definitely don’t need you to make the decision for them. Additionally, I’d wager it’s not doing your traffic any favors either. So stop it. Please.
Sometimes when reading something with a link somewhere, visitors want to open it in a new tab and sometimes they want to stay in the same tab, depending on their mood, astrological sign, the type of site they’re visiting and how good the content you’ve been giving them has been. So for you to try to hang onto their visit to your site after they read the thing you linked by messing with their browser, and browsing freedom, with an href target= attribute, whether _blank or new, to leave your site open when they close the linked site does not make for a better web (it’s by far more of an irritant than a convenient service you’re providing). If it were better for the web then browsers would do it by default.
In terms of your traffic, with good SEO one rule of thumb is that if whatever you’re doing makes the users’ experience on your site less loverly, that may produce a negative net effect, not help you. Do you prefer sites that link like this? Any idea off-hand how to override these instructions in your browser without digging through about:config or making your own greasemonkey scripts? No, right? You’d rather have the option to open in the same window, wouldn’t you, in your own surfing journeys? Of course. So stop doing it on your own site! In addition to pissing off your audience a little, you’re pretty much telling them that they’re not good with computers and need your HTML training wheels, and that you’re presuming for them that your site is worth returning to.
Want someone to come back to your site after they’ve departed? Produce better content. Step your site’s game up. Make it something you yourself would like to return to. Focus on the steak and not the SEO bullshit sizzle. Spending your time and energy (which is finite) by making a better site rather than pirating PDFs of books on secret SEO tricks is the key you want to use.
Hey wait, we’re not pulling any crap like this here, are we?