There is no such thing as Blue Shards, however if you meant "Blue Charms" I have provided you with a link below that should provide the answer(s) that you seek.
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The color shards you can find in Pokemon Platinum are Red, Blue, Green, and Yellow Shards.
you can find them underground
yes,
you can go underground if you have the mining kit and search there and you'll find all kinds of shards.
4 blue shards i think teach it to a golduck or psyduck holding the mind plate it does 100 i think !