blacklight_configuration_helper.rb

module BlacklightConfigurationHelper
  include Blacklight::ConfigurationHelperBehavior
  
  def default_sort_field
     if params.has_key? :q
        active_sort_fields.select { |k,config| config.default_for_user_query }.first || super
     else
        super
     end
     
  end
end

catalog_controller.rb

class CatalogController
...
   
   configure_blacklight do |config|
      config.add_sort_field 'relevance', default_for_user_query: true
      config.add_sort_field 'title', default: true
   end
end