I would first try a polarizer. It will darken the sky, and you might be able to control how much by the polarizing angle. You should probably have a polarizer, in any case, as it is a very useful filter.

A medium yellow, yellow/green or light green filter might work, too, if the polarizer darkens the sky too much. Yellow is perhaps the most useful filter, so again it wouldn't hurt to have it.

If you go to a local camera swap meet, you could probably pick up some used filters just to look through. Your local art supply store might have some colored plastic sheets that you could also use for estimating the effect.

If you don't have a black and white viewing filter, this should be your first purchase. It will give you a good idea as to whether or not you need a filter at all.