I've had DirecTV for the past 10 years and have never missed a C's game.
Here's how it works...
If the game is on "National TV" (i.e. NBATV, ESPN, ABC, TNT, etc..) then it is blacked out on Comcast SportsNet New England (CSNNE).
I much prefer to watch the game on CSNNE as I have a hard time listening to anyone that isn't Mike & Tommy, but it's just not possible when games are on National Television.
There are also a few sneaky things to learn. I believe the Detroit Pistons are showcased on FoxSportsNet Detroit, in which case CSNNE gets blacked out.
The first game of the Celtics/Wizards home and home back-to-back was on NBATV, so it was blacked out on CSNNE.
Any questions feel free to PM me anytime, but take comfort in knowing that if you have DirecTV you will never miss a C's game.