I have a bike with a rear rack and want to buy a rear fender. Is there any fender that is specifically designed to be used in a combination with a bike rack?
Fenders don't fall into an "ok with racks" or "not ok with racks" category. Generally speaking, most traditional fender designs will work on bikes with racks attached. There may be certain combinations of fenders and racks that are problematic, but those are the exception.
An example of a problem would be a bike whose rear rack is very close to the top of the rear wheel - it would be hard to fit a fender in there, but that's a specific problem which requires a specific solution.
That said, when putting a lot of accessories on a bike, we should expect to be bending and cutting and filing as we go. We should not expect a front/rear set of fenders and racks to be a perfect fit on a frame out of the box unless each of those parts were specifically designed to be a fit with each other.
Most standards fenders will work with a rack. You do have bikes designed for both - you will have two bosses. You can double up and use the same boss for both but a separate is much preferred.
If a bike will take a rack then it will (almost always) take fenders. But you can have a bike designed to take fenders but not a rack. People use fender mounts for a rack on bike not actually designed for a rack.
If its better integration of rack & fender you seek, look into getting a rack with a centerline stud or nut that you can attach the outmost half of the fender to.
See this pic for a rear fender+rack example.