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That is a sign that your STB is changing resolution when you go from HD to SD channels. It takes a few seconds for the signal to switch and for the HDMI handshake to be reestablished.
You can alter this if you choose by going to "SETTINGS>VIDEO SETTINGS>SD Override", if you select "Off" your STB output should remain the same as you have set for your HD channels, and you should not have the blackout.
The "down side" - if any - is that in this case, you are using your STB to upconvert the SD signals to HD, where many users believe that their TV or Receiver or Video Processor can do a better job of upconverting than the STB.
If you select one of the other settings (480i or 480p) the STB will pass your signal to your AV system in that lower resolution, and some other component will do the upconversion.
You shoudl try all of the settings out, and choose the one that provides you your favorite picture and performance.
I leave my box on SD Override "OFF" (unfortunately, it seems to change its setting magically at random intervals and I have to reset that selection rather frequently, but I'm used to dodgy performance from the Motorola machines.)