WOW, that would be hard to answer since everyones idea of an average blade of grass is different. I would think about the same weight as 3 average feathers.
What Is The Weight Of A Blade Of Grass?
To answer this question you would have to say "typical, average" grass blade for the common name or specific grass species name. Then the leaf area index and leaf (blade) width. The question needs better definition. Such as what is the average weight of the big blue prairie grass blade with a leaf area of 0.2, ~2mm width and 4ft length.