If you're using an averaging meter, pointing it towards the scene, which would be mostly sky, would give this type of underexposure. It considers everything you point it at 18% gray, same as a gray card. If the real scene is brighter than that, then voila, underexposure. Use either a gray card, or a meter with a 'dome' on it.
I have a Gossen LunaPro, and find I don't need a spot meter using the 'dome'. At least for roll film. Sheets is a different story.
- Thom