Up close (and back to looking brown), you can see the rayon (actually viscose) fibres wrapped with the cotton. The viscose adds a dimension of sheen and drape against the matte cotton, which does the job of holding the shape. Pure viscose would be too slinky and pure cotton wouldn't drape the way this does. The problem of course is when you blend two fibres like this, the yarn tends to be splitty. But not as bad as I was expecting, so it's worth it.
I really must find the bag of black Luna I know I bought - I'd make another sweater like this in a heartbeat.