Well, there is no front-end filter for that unfortunately. On the plugin back-end, under the Multisite options panel you can choose which blogs to include the results from, but of course that cannot be changed by the user.
Perhaps creating another search instance for a local search, or searching a specific blog only could be a solution. Not the best solution, but it could work.
Best, Ernest Marcinko If you like my products, don't forget to rate them on codecanyon :)