It is quite unlikely that a cure for the blight was found, since as you said yourself, they could have just stayed on earth and gradually recultivated it in this case. So the 1st option stays out of consideration. It is, however, likely that you could cultivate plants in a closed ecosystem once you have seeds separated from the blight, afterall they somehow would have to grow new stuff on whatever planet they land on anyway. So yes, the corn there was likely isolated from the blight still running rampant on earth.
But as to the idea of building closed ecosystems on earth, there was another severe aspect to the whole blight problem. It was slowly turning earth into a plantless desert with a toxic atmosphere. So the only possible long-term way to overcome this would have been to not only grow the food inside biodomes, but to move the entire human civilization into biodomes on an otherwise completely hostile planet. And I'm not sure this would have been a remotely possible endeavour on the long run, or at least not as an ultimate solution for humanty's progress. Granted, it might be sufficiently reasonable to serve as a last resort of hope, hadn't we found that promising wormhole, which suddenly made an even less likely solution much more reasonable.
But I admit that I'm not providing a definite answer here either, only possible ways to explain the situation. The idea of moving humanity into biodomes might as well just not have been considered by the writers at all, since of course it wouldn't make the humans strive for new dimensions and a better future, but make them effectively prisoners surrounded by a dead and destructive world waiting for their slow demise.
Another possibility might be, that this in the end wasn't real corn afterall, as this whole area was practically just a museum to remind humanity of its (and especially Cooper's) past as farmers.