It's known that Roxas's role in the organisation is to destroy the manufactured heartless (the ones created in hollow bastion) and collect their hearts.
Well in kingdom hearts II when DiZ destroys a large section of kingdom hearts releasing the hearts. The hearts fell to create pure bloods.

I may be over thinking this but doesn't it seem weird that the manufactured heartless could turn into pure bloods?

once the manufactured heartless are destroyed, they're gone, just like normal heartless, but the heart remains. A heart is a heart, no matter what. Since TWTNW was a world with nobodies AND pureblood heartless, the darkness would have consumed the hearts and the shadows were born, but what I don't get is why couldn't the dusks who were trying to get the hearts become human?

Well, they'd have to find their heart to become one again; a random Dusk couldn't just grab the first Shadow it saw and return to being a whole person. At least, not the same person they were before, if mismatches are even possible.

It's also possible that lesser Nobodies like Dusks lack the intelligence necessary to actually go look for their Hearts.

Actually,

A Heartless is made from the Darkness in the Heart.

Sora had very little Darkness in his Heart.

To get Evil from him would be like wringing water from cheese.
It'd be amazingly hard,
And you'd only squeeze out so much.

That's why he was a Shadow.x3;;

quote Khnurdlul

Actually,

A Heartless is made from the Darkness in the Heart.

Sora had very little Darkness in his Heart.

To get Evil from him would be like wringing water from cheese.
It'd be amazingly hard,
And you'd only squeeze out so much.

That's why he was a Shadow.x3;;

That's right, but the embelemed heartless are created in hollow bastion (confirmed in the secret reports (can't remember which game)). Only the pure bloods are formed from the darkness in humans blood.