I will add a property on the SearchInput control for FeatureGuid. If specified then the feature guid will be passed as a query string parameter to filter results to only that feature, similar to the feature dropdown list on the search results page which filters results by feature.
This will be in the next release so you would be able to add more than one SearchInput to layout.master if you like and set a featureguid on it if desired. Probably what you would want to do is use a different skin on the blog page and set the search input there to have the guid, it would probably be confusing to users having more than one search input.
FeatureGuid="026cbead-2b80-4491-906d-b83e37179ccf" is what you would put on the control to search only the blog feature. I've just tested it here on my local machine.
Hope that helps,