The small blue forget-me-not flower was first used by the Grand Lodge Zur Sonne, in 1926, as a Masonic emblem at the annual convention in Bremen, Germany. In 1938 a forget-me-not badge—made by the same factory as the Masonic badge—was chosen for the annual Nazi Party Winterhilfswerk, the annual charity drive of the National Socialist People’s Welfare, the welfare branch of the Nazi party. This coincidence enabled Freemasons to wear the forget-me-not badge as a secret sign of membership.
After World War II, the forget-me-not flower was used again as a Masonic emblem in 1948 at the first Annual Convention of the United Grand Lodges of Germany in 1948. The badge is now worn in the coat lapel by Freemasons around the world to remember all who suffered in the name of Freemasonry, especially those during the Nazi era.
(the source/read more: Wikipedia)