Hi, there are a couple of settings you van check:
1. Global Illumination, make sure you turned on the secondary Method, use irradiance Cache and up the intensity try around 250
2. Stochastic Samples, go to customize and put them up to 128
3. You really need more light in there, as example add a light behind the camera. Imagine this in real life, if you had just this one light it would probably still be too dark. I say this as more light helps getting rid of the blotches too. So you could lower the samples again which will render faster.
Hope this helps.
AC 3 to 26 -Windows 7 - 24GB - 8core 4.4Ghz