When my brother and I ride together will wil do this. Usually when there are only passing 'spaces' for one. I can count on one hand the number of times we have done this on a truly blind curve, in over 20k miles of travel. The rest of the time it is usually a notice, so the other rider can start accelerating before the road can be seen well. And, yes, one time it did startle the driver of the other vehicle a bit. They hit their brakes when they realized I was going to pass.
We also communicate road conditions. But, this is kind of a moot point since the person in front is riding slower anyway. Most of the time we're just talking about scenery, bike conditions, where to get gas, next stop, next intersection, etc.