I really wouldn't suggest silk as the main fabric for a 'darksuit', to be honest. Silk has, from what  I recall, this tendency to be shiny.

For thieves, this is a /bad/ thing since shiny things reflect light and can get them caught, killed or both.
Also, you mentioned that you don't want the suit to seem out of the ordinary; asides from being shiny, silk tends to be expensive and unless the clothes your thief /normally/ wears suggest that such a cloth is within his price-range, I wouldn't use it. Heck, I wouldn't use it anyways, just because it's /shiny/ and when you're sneaking around in the dark, shiny things are /bad/, since they reflect torchlight, moonlight, starlight - you name it, they're probably going to reflect it.

I'd suggest velvet, for preference, dyed cotton or something similar. Not silk. However, if you were to use Eagleon's idea, you /could/ use silk as the inner lining for the cloak, since as he said, something drastically different from the norm may throw off pursuers. But just the same, I'd advise against using it for the majority of the darksuit.

Also, for the pants, you could include a hidden pocket for a knife.