The Blue Feather is a type of tool in Harvest Moon: Back to Nature.
The Blue Feather is needed to propose marriage to a bachelorette. It becomes available at the Supermarket for 1,000G when a girl reaches an affection level indicated by an orange heart. Alternatively, when you are carrying 30,000G, Won will visit the farm and offer to sell the Blue Feather for the outrageous price of 30,000G.
Although the blue feather is used to propose, a bachelorette will not accept your proposal if you have not met all the necessary marriage requirements. These include: