They look like Concord grapes to me. If they are Concord, they can be used to make great jelly. A Concord grape, when ripe, has a sweet tasting skin and a bit tart flesh inside around the seeds.
Maybe your grapes are vines that have gone wild over time, aren't getting enough sun light to be really healthy due to tree growth, just sort of 'hanging on'.
Did/does the property have any old houses or cellar holes that would indicate someone may have kept a garden years ago?
Dave.