On harvest moon for the SNES how do you get the blue feather?

That depends on what Harvest Moon Wii game you are playing.

Tree of Tranquility:

Once you get a marriage candidate to eight hearts you will be told the Legend of the Blue Bird.

A few days after that you will see a Blue Bird flying. Go to the Waterfall area to find a fallen Blue Feather.

Animal Parade:

Once a marriage candidate reaches nine hearts leave your house on a Sunny morning and Mira will be on your farm. She will tell you she was following a Blue Bird and then tells you about the Legend behind the Blue Feather.

After that scene go to the summit of the Mountain between 5pm to 7pm (17:00 to 19:00 on the Animal Parade clock) on a sunny day to get a Blue Feather.