I know I can Command-click it to achieve that. What I want is to change the default behavior from opening in the same tab to opening in a new tab.
Is there an extension or a hidden configuration for that?
In this article Jonathan Rentzsch creates a new service because he wants "Search with Google" to work in Chrome (his default browser) but a side benefit of following his tutorial is that your search should open in a new tab.
If your default browser is Safari it should open your new tab there.
Warning: This is essentially a follow directions, cut-and-paste affair but it does involve seeing code. I'm unaware of a easy to install extension to accomplish this same task.