Are antidepressants potent antihistamines also?

Some are, yes.: Looking at receptor inhibition profiles (lab data) we can see that some do indeed block histamine receptors. Older ones in the tca class (elavil, anafranil, etc), trazodone and even a more modern one like Remeron (mirtazapine) do so which can lead to increased sleepiness and appetite. As far as potency, they're not as effective as sudafed in fighting a cold.

AD's: Some antidepressents have anticholinergic/antihistaminic effects.