While driving in temps that cold is it recommended that you block air flow over the radiator so that the engine can reach a proper operating temp?

Did the OP do that while driving that day?

About ten days ago it was in the single digits F around here and I noticed that my heater wouldn't ever really get as warm as it should.

Last year in my W123 I covered the radiator with a piece of cardboard under the grill and it remedied the problem.

The only problem I had was keeping it on there one day when it reached 60F and it almost overheated....

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1993 W124 300D
-297K on the clock as I type this.