There are two different problems here:
1. You may get a steady black screen after switching channels. Presssing ChUp and ChDn usually takes care of this. This is definitely a bug in the STB software.
2. You may get a temporary black screen while watching a channel. This seems to happen on select channels only. (I see this often on HBO). This is likely to be an uplink/transmission issue. I don't think this is a bug in the software, as it seems to be channel-specific.