+1!!I use my pressure cooker ! Skin on. The skin falls off after they are done, 15 mins.
Perhaps you just didn't have enough water to steam them with?
Sweet potatoes, like white potatoes can change colour when exposed to air.
No, the idea of steaming is that they are not submerged in water. But enough water to create the hot steam which is what will cook them. Were they dried out when you finished them?