Answer:
   
   First, note that both Mother's Day and Father's Day are American
   holidays, not Jewish holidays. Although some congregations may
   recognize them, they are not Jewish holidays, as demonstrated by their
   being observed on a Sunday, a traditional Christian day for church
   worship.
   
   So, how to bless your mother. Listen to what she says; do what she
   asks. Find a nice Jewish person to marry.
   
   Seriously, although one may be able to develop a blessing for
   anything, in traditional Judaism, blessing one's parents is not the
   normal construction. In traditional Judaism, blessings are generally
   bestowed by the have to the have not: kohein to masses, Abraham's
   children to the rest of the world, parent to child, rebbe to
   student/chasid. To bestow a blessing implies having G-d's "ear".
   Everyone has G-d's attention, and "The blessing of commoners should
   not be a light thing in your eyes." This adds much meaning to wishing
   another "Mazal Tov!" or "Refu'ah sheleimah" (complete healing).
   However, codified blessings tend to run in one direction. A creative
   rabbi, of course, could craft something, but it wouldn't be a codified
   construction (i.e., standard in Judaism).
   
   By the way, what is a good day in Judaism for recognizing parents?
   We've noted above that Mother's Day and Father's Day are not
   (actually, they were created by the greeting card companies). Here's a
   suggestion: Shavuot is a great time for children to honor their
   parents, as the Torah portion for the week is a reading of the 10
   commandments that includes the directive. It is also a great time for
   remembering all the other 9 commandments, and that we should be
   following them (as we do every week when we study Torah).