It's possible that a garment or rag with bleach on it got mixed in with your t-shirt. There is no way to return the blue color, that I know of. However, there is one creative solution: Buy a bleach pen that's used for fighting stains. Use it to make white lines and dots on the shirt, then wash it by itself. You'll end up with a t-shirt you designed yourself and you wont have to throw it away.
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