No.
If it ain't there you can't fool the camera into thinking it is if ND,
polarizers, UV filters and exposure adjustment won't work.
Learn to use Photoshop to fake it.

Anything that darkens a light, pale blue sky will increase its saturation,
making it appear bluer.  Therefore polarisers and ND grads will actually
make the sky bluer.  Another option is to simply expose for the sky, though
this will sacrifice the rest of the photo, most likely.  The idea is to get
a large difference between the blue and the red channels - the more this is,
the higher the saturation.  Well, that's the theory - tell me if it works.

If the sky really is overcast, and definetely not even slightly blue, then
I'm at a loss at what to do, apart from putting a blue grad filter and not
worrying about clouds looking blue.

Duncan.

You could try shooting Tungsten film under daylight conditions. Of course,
that would make everything more blue, though you can knock that down a little
with a slightly orange filter (not strong orange).

Weird Cokin coloured grad filters. The problem is that the have a somewhat
definite edge, and that might show up in the images. Definitely low cost
enough to try out one or two.

A really strange way to do this would be to cross process some colour
negative films in E-6. Depending upon the film, the blue effects will vary.
The other colours will also go a bit off as well, so unless you want a
slightly surreal look, cross processing might be fun, but not a solution.

Ciao!

Gordon Moat
A G Studio
<http://www.allgstudio.com>