A papal blessing is available for new marriages, 25th, and 50th anniversaries in the form of a certificate. You can apply for those through your Chancellory office in your diocese. I don’t know if they are available for 20th. You also have to apply BEFORE your anniversary date.
A papal audience is different from a papal blessing. The papal audience on Wednesdays is open to the public, but you have to get a ticket to attend (free, you have to go to the Vatican ahead of time and get the ticket).
There is a special section of seats for newlyweds-- that requires a different ticket and that is obtained by going through your diocese and the North American College in Rome. Newlyweds have to have proof of their marriage in the church within 2 months-- my husband and I did this.
I do not know if any similar priority seating is available for 20th anniversaries. Contact your diocese or the North American College in Rome.
Also, you don’t get an individual blessing by the Pope at the Papal Audience, there are thousands of people there.