In order to get a lossless (from the cable box to your PC) capture you'll almost certainly need a cable card tuner, ie a Hauppauge WinTV-DCR-2650 or Ceton InfiniTV. Then you need to hope that Comcast has that channel set to "copy freely". Then, theoretically yes you could burn a Bluray disc (why you would want to with magnetic storage being so cheap, I don't know).
There are also some cards out there that will record anything from component inputs, then re-compress the video to h.264. That's lossy, but still HD. Cheaper cards that take composite video, will not be HD. Digital (without cable card) tuner cards will only be able to record OTA or in some cases clear QAM, but its very unlikely the channel you want to record is available on clear QAM. Unless you want to record NBA games on ABC.