'96 Blazer Heater Core, No Heat?

Dear Gang, Help! My '96 S-10 Blazer 4 door, 4 wheel drive. 4.3 liter engine/a.c. has no heat. Vehicle was driven into snowbank last Feb. 2003. Had heat up till then. Dex-cool fluid had been very brown up till then. It had been (Jan. 2003) flushed by Automotive chain, not a very good job. Plus the system was not purged of air pockets and she did get hot (temp. gauge) on the way home. Stopped and put more dex-cool in the radiator to top the system off. I did flush the system myself, Dec.2003 with Prestone Flush Tee kit and Prestone flush mix and put new dex-cool into it. Not the Heavy Duty mix. Still no heat. I did read bowtiebandits diagnosis and procedure for flushing which I am going to do. Question? Is it possible that the flap valve (dash switch which controls which way the hot air flows) that opens next to the heater core could have closed form the impact of driving the the vehicle into the snowbank? Me myself I am leaning 90% towards bowtiebandits diagnosis. What do ya'll think? Cold in South Jersey. Thanks, Brian.