The Finnish blue swastika was originally the personal emblem and good-luck charm of Swedish Count Eric von Rosen, an explorer and adventurer who donated his Thulin Type D biplane aircraft to the White Finnish government on 6th March 1918. He had painted blue swastikas to the plane's wings as personal good luck charms, and these were retained as the plane was used in Finnish service.
This was the first aircraft used by Finnish military forces and the Finnish Air Force counts its establishment from that date. The emblem stuck until the end of the Continuation War, when for obvious reasons it was changed to a more neutral roundel that is still used.
The relationship to the swastika used by the Nazis is incidental. Von Rosen was later active in the Swedish Nazi movement, but the Finnish symbol predates the Nazi party and its use of the swastika by several years.
The swastika was also used as a national symbol in Finnish tanks of the period, albeit in a slightly modified form, and in many other heraldics from the post-First World War period. It can still be seen in the FAF heraldry, for example in the flag of the Air Warfare School here:
There are also certain decorations that feature the swastika.
