Until they provide STB's that generate test patterns from the box itself, calibrating cable (or satellite) sources will be hit or miss.
Program material is handled inconsistently from one channel to another or even from one show to another, so one calibration may not give great results in all situations. And honestly, when I'm watching a typical bit-starved low-pass-filtered frame-dropping SD encode of some old B movie, the last thing I will be complaining about is a badly "calibrated" input on my display.
Still, for those times when the source is very good, adequate bit rate is provided, and standards were followed at every point along the chain, you do want to have the basic settings in the ball park.
Generally, if you have calibrated your display using one HDMI source, you can try to use the same settings when viewing another HDMI source provided the two devices aren't using completely different output settings like "video" vs "PC" levels or RGB vs YCbCr color spaces.
A very quick and dirty technique for checking black level is often described as follows: find an HD channel showing 4:3 content surrounded by black pillar bars. Pause a scene with relatively low APL. Adjust your black level (aka "brightness") to its minimum, then raise it until the black just begins to visibly change to gray in the pillar bar area of the image. Go back to the level just before this point, the highest possible setting where the pillar bars are still completely black. Again, you may want to repeat this at different times using several different channels to make sure you didn't happen to use a non-standard broadcast for your test.
Once upon a time (as recently as May 2011), HDNet was broadcasting a 10 minute "show" displaying test patterns on weekend mornings (eg, Saturday, 6AM ET / 3AM PT) . If they ever do so again, record it, and do your best to use those patterns to calibrate. Here is one man's opinion on how to use the patterns: http://www.smartcalibration.com/hdnetpatterns.html
I am not sure I understand how you would play the "MP4 files" directly from the Comcast box? You basically have to have a source that came into your building from the cable network. Once you have it saved on your DVR, you can be pretty well assured the DVR is not going to do anything more to it.
But I don't think cable (or satellite) STB's "play back" content other than what originated from their networks.