if it's from natural causes, the only thing i can think of is hardened lava, which is cooled by ocean waters, and then bits of it scattered throughout the beach. that's how some "black beaches" are formed, iirc.

but if it's related to pokemon, i thought of darkrai, or possibly other pokemon who can lure people to sleep. but i can't think of how it would be connected to black sand otherwise...

i'm wondering if this portion is the part to focus on

She remembers... a blue or purple light, and the wind picking up a little... a voice in her head? But she can't remember what that voice said, and whatever happened after that, she can't remember, and then she woke up here in the hospital