Brown spots during pregnancy is normal. Just to keep you mind at ease just show it to your doctor at your next visit. Good luck.
Are you sure it's just the skin that's darker? Or could it possibly be where a vein problem? If you aren't sure, you should ask your doctor to take a look.
Spots of darkening skin are normal with pregnancy. My belly button turns nearly black (I'm white) during pregnancy. Your skin should return to it's normal some time after birth. It has something to do with your hormones, but I can't remember what exactly - I'm sure someone else can tell you exactly how/why it happens.