It isn’t showing “partiality” to call Mary “Blessed”. It’s quite Biblical.
“Blessed art thou among women...”
If, in the Bible, an angel calls Mary “blessed’, we can’t err in doing so as well.
It seems quite convoluted to claim that in doing so, one commits the sin of partiality.
And yet look how you treat St. Pete in direct contrast!
Matthew 16:23
Jesus turned and said to Peter, "Get thee behind me, Satan."
We're all blessed in Christ.
Ephesians 1:3-4 3 Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places, 4 even as he chose us in him before the foundation of the world, that we should be holy and blameless before him.