As I'm currently in a metalworking class I'd recommend picking up some Liver of Sulfur, putting it in hot but not bubbling water (low heat). It will slowly darken until it's completely black. You can then use a rag or shammy to lighten parts of it while leaving the deep grooves black.
1. Sterling silver can be darkened by placing it in household bleach (doesn't always come out as black as with liver of sulfur though), or you could just heat the piece up with a jewellers torch and get the surface to to oxidise.
2. To take the shine back a bit on a highly polished piece you can give the model a bit of a rub with a Scotchbrite.
3. Or you could satin finish the accessable parts of it by using emery paper.
4. If you've got access to a sandblasting machine this can also achieve a matte finish.
5. Using jeweller's drills and diamond drills, the surface can be textured in various ways
I agree sometimes household bleach can get some unexpected results...brownish, reddish etc
I used some 'White King' bleach yesterday and got quite a nice dark grey colour on one of my Shapeways models I'll upload photos soon.
It's not quite a combination of matte and polished but here are a few techniques I applied to a sterling silver key that Shapeways produced for me....1. some extra polishing, 2. Household bleach, 3. Scotchbrite, 4. kitchen scourer
I uploaded images to unellenu.com -> techniques -> finishing