I used to use a deep 25 red and a polariser when shooting Kodak Tri-X and Ilford HP-5 to deepen the 'grey' of blue-skies. Gives a very 'John Ford western' look to landscapes. You can turn a blue sky almost black with that combo.
And if you don't know what the hell I'm on about, shame on you - go rent or buy the John Ford cavalry trilogy (Fort Apache, Rio Bravo and She Wore a Yellow Ribbon) starring John Wayne.