The B&W 3x6" Graduated Blue #581 Glass Filter is an excellent solution when the sky isn't blue enough for you. It can be used effectively in combination with another graduated filter for the lower half of the image, so that in such a combination the sky will not be rendered too bright or too pale.
For example, a photo of sand dunes by the sea; Using a B&W Graduated Tabac Filter for the lower half and a B&W Graduated Blue Filter for the upper half of the image.
Excellent choice for adding more "punch" to sky color while leaving bottom half of image untouched