Google initially launched the gadget on its Blogger in Draft platform, which offers users a version of Blogger where Google tests out features and new interfaces. Google says it has upgraded the search gadget to provide simpler defaults as well as the ability for the box to integrate with the aesthetics and color of your blog. The Search Box gadget uses AJAX Search APIs to power the feature and also automatically updates the custom linked search engine when you update your blog, blog lists, or link lists.
Custom search can be a useful tool for blogs because it allows readers to not only search a blog’s content but also any pages or favored sites that are linked to, giving readers a related, but still-focused, search experience.
You can activate it by going to your Dashboard > Layout > Add a New Gadget and add Search Box (New!)
This is how a search performed on your blog will look like: