This is how Blue Dragon World has been taken over by Darkness in Kingdom Hearts X (Chi) Back Cover.
Shu: After me and my friends find out about the rumour, and that day before our Journey begins... Darkness has arrived.
Shu is sleeping, and wake up and wake up and find nobody's here in the village
Shu: Grandpa? Jiro? Kluke? Anyone? Where are you? I wonder where they went?
He went outside and he look shock, because he saw a Giant Ball above
Shu: What the!? What is that thing!?
Then he saw Shadow from his Dream
Shu: Oh no!
He went off to find his friends and he saw them
Shu: Guys! There you are! What's going on?
Kluke: We don't know, and we do know that there is Trouble around the Kingdom of Jibral
Jiro: And we thought, Nene is back.
Shu: But where did everybody go?
Kluke: We don't know, The King of Jibral is not here.
Jiro: My parents isn't here.
Marumaro: Lady Zola, isn't here!
Kluke: And even though, everyone is gone.
Shu: Where's our friends?
Jiro: I don't know, where they is. But we have trouble from them.
They saw the Shadow
Shu: Them again!? Let's fight!
They are fighting them with their Shadow and then there are too many of them, and they surrounded them
Kluke: We're surrounded!
Jiro: Oh no!
Shu: We can't give up!
And then the Shadow is gonna tackled them
He hold her to protect her from the Creatures
Then a bright light has save them, and then they have the Digivices and weapons
Shu: What happen? And what's this thing, I'm holding?
Jiro: I don't know, but are these weapons? I got a Sword like you, Shu.
Kluke: You're right. I got a Wand.
Marumaro: Maro got a Shield!
Kluke: But how did we get one? And what are these device, we're holding?
Shu: I don't know, but we have to go find Zola!
They went off and they saw Zola
Zola: Shu. Jiro. Kluke. Marumaro.
Then a burst of Darkness has blown our Heroes away, and they landed on a floating mountain and they saw everything collapsing
Jiro: What's going on!?
Then a Huge Shadow has appeared, they are fighting them and they defeated it and then Shu has gone Unconscious
Then the huge Shadow has been suck into the Giant Black Ball, and then Shu has been drifting away, Jiro and Kluke are gonna save him and they did it, and now our Heroes has been sucked into the Giant Black Ball