Your issue is not caused by signal. A weak signal causes pixlation or simply blank video, not blurriness. This is digial, not analog TV.
I can't tell what's wrong with your picture. But if you use a PC to playback HBO directly on your TV, you can encounter two problems:
1. 29/59 bug. HBO channel almost always exhibit this issue. Search 29/59 bug. The typically symptom is contrast goes up and down all the time or a choppy video.
2. Limited vs Full range issue. That's a configuration issue. A mismatch could cause washed out color or too dark of color
As others have already pointed out, you could also be looking at the wrong channels (SD channel vs HD channel). I know some cable boxes can automatically map the HD channel to where the SD channel is if you are connected to an HDTV via HDMI. But a CableCard tuner won't do this. Typically, SD channels are at lower channel numbers and HD channels are at higher channel numbers. For example, my cox system has all SD channels below channel number 1000 and all HD channels above channel number 1000. For most channels, they have both SD and HD channels. Typically you add 1000 to the SD channel to find your HD equivalent.
So, check the channel number on your tuner and compare that to your cable provider's channel line up.