'Kalu' means black in Sinhala: the near-black colour comes from jaggery cooked down with coconut milk for hours, not from any added colouring.
Rice flour, coconut milk, jaggery and cashews are stirred for hours over a wood fire into a dense, near-black, gelatinous sweet, cooled into fudge-like slabs and sold sliced by weight from stalls and home producers around Galle and Hambantota.
Why it's made this way: The name and method are usually linked to Malay influence, though its exact origin is unclear; it took root along the southern coast, where Sinhalese, Moor and Malay cooks alike still make it for occasions like Sinhala New Year and Eid.