How can I get a Blue Feather?

Ah, good ol' harvest moon.
The original versions were the best.

You ahve to bring a marryable character to the Orange heart level.
(In older versions it was just alot of hearts)
You should be able to check their heart level when you talk to them as a heart should be on their chest, I think.

Once that thing is orange you should be able to find the blue feather for sale at a hefty price. Until then you won't see it.
(in older games it was a tad harder, and you got it from an odd merchant)

Some characters that can be married have invisable heart meters too, so it'll pay off to keep being nice to characters you really like. You just won't be able to know when the time to marry is, is all.

Once you have that feather, well, you should know what it does by now. :P

Good luck and have fun.