Seems to me that Jesus had the perfect opportunity here to confirm the subsequent Catholic myth of the "blessedness" of Mary. Instead, He directly contradicts that view.
Please!
"Blessed is she who has believed, because the things promised her by the Lord shall be accomplished." (Luke 1.45)
As St. Elizabeth said there, Mary is Most Blessed because she perfectly conformed her life to the Will of God for her and never sinned. That is the import of Luke 11.27-28 too. Jesus explicitly says Mary is blessed not for giving birth to Christ, as exalted a task as that was, but because she heard the Word of God and obeyed it.
As an aside, in the Tridentine Missal, Catholics use Luke 11.27-28 as the Gospel at the Mass "Salve Sancta Parens", the first Common Mass of the Blessed Virgin Mary on Satuday. It is also the end of the Gospel for the Third Sunday in Lent. You seem to think we would not be familiar with it though? Perhaps you don't understand that we have a lot more reading of the Bible at our Masses than you do at your prayer services.