I have a older ATI TV tuner USB stick - the ATI 600 TV Tuner.
The blacks aren't great on it. It looks pretty washed out in comparison to other HD sources like a bluray, but it is HD, and it has smooth playback, and supports Dolby Digital - so it's not terrible. I use it exclusively for over the air tv tuning. Are the internal cards improved? How much does video quality differ from TV tuner card to TV tuner card? What's currently recommended most highly for over the air use? I'm feeding the USB card through a GTX 670 to an Epson 8350 and the regular computer display doesn't have the same washed out blacks when playing input from say CyberPower DVD etc.
It could be your tuner but most likely it's a video card setting that needs to be changed, such as full range color or limited range. And if your using WMC it could be interacting or working against your video card settings.