I would give them circumstance bonuses to hide, and be suer you take distance into consideration for spot penalties. Once in combat, I might give them 20% concealment at best for melee. For ranged combat, I would probably consider giving them more concealment.
Darkvision does black & white, but I figure that a black pudding isn't an exact color and consistency in color. In normal light, it is very dark, and appears black. But, in contrast to obsidion, or black paint, maybe it is more a deep brown (Chocolate pudding!) or something like that. I also tend to think of darkvision as providing varying shades of gray, but that might be more me than the book. In any event, once you have something eating your leg, you are just looking for contrast to figure out where to hit it. That is why I wouldn't provide real heavy concealment bonuses in melee.