Don't think it's Nanogona polydesmoides - doesn't look like a Craspedosomatidae to me, but much rather like a normal Polydesmidae - something like Polydesmus angustus maybe, but - for nincompoop me - impossible to tell from such a small image, especially without a size estimate.
Archispirostreptus gigas, the giant African millipede is one of the largest of the millipedes, growing up to 11 in (28 cm) in length. It lives in tropical and subtropical Africa, in rotting plant life or moist earth, and usually avoids light. It is black in colour, and is often kept as a pet.
This looks to me like a Polymicrodon polydesmoides but the photo isn't quite large enough to see it accurately so it could be Oxidus gracillis. As you said its flat backed though its probably Polymicrodon polydesmoides.