You need to max out a pokemons beauty by feeding it lots of high level blue pokeblocks, then you need to talk the president of the Pokemon fanclub in Slateport, he will admire your Pokemon and the effort you put in as a trainer and then give you the blue scarf.
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im not so sure... but i think that when u go to the Pokemon fan club in slateport city and talk to one person, ur first Pokemon in ur list should be good in beauty contests for blue scarf, smart for green scarf, cute for pink scarf, cool for red scarf, and tough for yellow scarf. or at least that's kinda how it is in ruby and sapphire, except that they can only get one scarf.
There is no beauty scarf in Emerald. However there is a blue scarf (raises beauty) and it can be obtained like other colored scarfs. First, max a Pokemon's beauty (anyone will do). Then go to Slateport City (by boat, fly, cycle whatever). Then go to the Pokemon Fan Club. Talk to the chairperson inside when you switched the Pokemon with max beauty to the first Pokemon. He will say that it is well trained or something close and give you a blue scarf. Likewise, maxing out a Pokemon's coolness and putting it as the first Pokemon will allow you to get red scarf (raises coolness).
Get a Pokemon to max out beauty (preferably feebas since it can evole when getting it and training at least one level) by feeding it pokeblocks. Pokeblocks can be made in lilycove. Go to Slateport and talk to the Chairman (if i remember right) of the Pokemon Fan Club with the Pokemon that has max beauty in front. He will give you a blue scarf after the conversation.
You can acquire a Blue Flute in Pokémon Emerald Version by travelling to Route 113 and walking 250 steps in the ash grass. Then, go to the house which is along the route and talk to the man inside. From there you can get the Blue Flute.
talk to Wallace