<root>
  <!--blahblah-->
  <child/>
</root>

Nokogiri::XML::Builder.new do |xml|
  xml.root {
    xml.comment('blahblah')
    xml.child
  }
end

<root>
  <comment>blahblah</comment>
  <child/>
</root>

require 'nokogiri'

xml = Nokogiri::XML::Builder.new do |xml|
  xml.root {
    xml << '<!--blahblah-->'
    xml.child
  }
end

puts xml.doc.root.to_xml
#=> <root>
#=>   <!--blahblah-->
#=>   <child/>
#=> </root>

class Nokogiri::XML::Builder
  def comment(string)
    insert Nokogiri::XML::Comment.new( doc, string.to_s )
  end
end

xml << "<comment>#{comment}</comment>"