RE: someone's in MY blind
blind ownership depends primarily weather it's on private or public land and the laws regulating waterfowl hunting in your state. if it's on private land your usually trespassing if you pass the high tide water line and thus can't use the blind if it's on shore, if it's an offshore blind off private land it is in my opinion it's rude to use it.
if the blind is on/off shore on public land in a state that does not require the registration of blinds than in my opinion you could use it but be nice if the guy who built it shows up to hunt it.
in maryland all blind locations on/off shore have to be registered with the state. landowners or designees have first dibs to register their shoreline then it's opened up to the public on a first come first serve basis, no blinds are registered that are closer than 250 yards from each other, and it's a crime to hunt waterfowl anywhere other than your registered blind spot or private shoreline without written permission of the landowner or public guy who registered the site.
the dnr will remove and ticket someone hunting out of a blind location that is not registerd or they do not have permission to use.