I searched for it and nothing but a bit dodgy sites that I don't trust and with nothing on the COD website or steam it makes me even more suspicious. Just makes me also think they might be hiding the demo so you just buy the game instead?
I don't think there's a demo out there tbh. The game's a good one anyway, even if it is buggy as hell.
If you have MW2, you can simulate Black Ops by cranking the settings high enough for the game not to run properly and by imagining a couple of new maps. Really it's exactly what you might expect - in good and bad, no need for a demo here IMO.