Something I've noticed is when you add a bump to any texture on anything that has folds/creases/indentations and then render it on the default render settings, you get that crazy black stain all across the area (its especially bad when ambient occlusion is on as well). What I usually do is set the bump down to anywhere between .001 and .005 and then customize my render settings (Irridance caching: between 70 and 90 - Pixel samples: 8 to 10 - Min. shading rate: 0) Thats my standard render setting.

Hope this helps a bit :)

Ana