Yes that would work...
The blond lady in Floaroma Town will only give you the Gracidea Flower if you have a specific Shaymin in your party meaning your Shaymin has to be either an event Shaymin or the Shaymin that was caught in the Flower Paradise in Pokémon Platinum, no other Shaymin will work not even ones that were hacked.
The reason why showing a Shaymin to the blond-haired lady near the sign in Floaroma Town wouldn't give you the Gracidea Flower is because it has to have a Fateful Encounter flag such as the Shaymin that were given out at the movie theater in Japan or at various stores in non-Japanese countries or even the Shaymin that could be caught in the Flower Paradise in "Pokémon Platinum."
You won't get it. you can only get the Gracidea flower in Pokemon Platinum. If you got the shaymin from diamond and put it in platinum it wouldn't work, you have to get a event shaymin from mystery gift or go to toys'rus at certain times. You can also get it in HG/SS in the flower shop in goldenrod city. that also involves the event shaymin.
Go to a girl in Floaroma Town and get the Gracidea Flower from her. Give the Gracidea Flower to Shaymin for it's sky form. The Gracidea Flower does not work at night.
If this is in Diamond or Pearl, it is impossible to receive the Gracedia flower or Shaymin Sky Form. In Platinum, you must have a Shaymin IN YOUR PARTY and then ask the girl. She should give you the Gracedia flower, but it does not work at night or on any other Pokemon.
You CANT but you could get the gracidia flower.Talk to the lady in flamora town if you have shaymin in your party she gives you the flower then use it on shaymin and it will turn to sky forme.ps dont trade the shaymin from a diamond or pearl game and try to get the flower i tryed and it didnt work.
It's possible that it's not the Gracidea Flower that isn't the problem but in fact the Shaymin you're trying to use it on that is the problem. The Gracidea Flower will only work on legit Shaymin and not ones that were acquired via cheating or hacking.
You need to get the flower, by taking a LEGIT shaymin to the flower shop, talking to a certain girl, who will give you a flower of which you must use on the shaymin. It will work if the shaymin is LEGIT.
Shaymin reverts to its normal form at night. Wait until the day to make it sky forme again
event
You need to have Shaymin in your party while doing this. It only works if the Shaymin was given in an event or you used Oak's Letter to catch Shaymin in it's natural habitat in Flower Paradise so if you just hack it off using a cheating device, it won't work. As soon as you have Shaymin, go to Floaroma Town and talk to the girl near the sign. She will give you a Gradicea Flower. The flower is used to turn Shaymin into Sky Forme without trading it into Pokemon Platnum when you use it on Shaymin. And since it's a key item, you can turn it back it into Land Forme if you want to.
You need Pokemon Platinum and the TRU Shaymin in your party. Go to Floaroma Town and talk to people in town. One of them has the Gracedia, Use it on Shaymin. Shaymin goes back to land form at night. Shaymin also goes back when it is frozen.