The issue with New York was that there was SO MUCH time travel going on, and so many paradoxes (paradoces?) occurring at once that a mass of chronal scar tissue had built up. Rory seeing himself, the bootstrap paradoxes of the book and River's message in the pottery, all typing up with the massive "grandfather paradox" of Rory and Amy killing themselves, thus preventing themselves from ever being able to see an older Rory, a paradox so massive it broke the whole chain of events.
That moment of time in that area is being held together with this thinnest wisps of spacetime, and the disruption to the time field caused by a TARDIS would shatter the thin tissue.
However, a vortex manipulator is apparently perfectly safe, as River uses one to go back at least once, and presumably several other times just to check in on everyone.
As for Blink, there IS no paradox. There is evidence in the present that The Doctor was sent back several decades. All of that evidence was created before the point that Sally was able to send the TARDIS back with the Emergency Recall Program on the DVD. So The Doctor leaves after leaving all the breadcrumbs; after that, there's no actions or events that don't get created by his departure. If he'd decided to go back FARTHER to eliminate his meddling, that would have caused a paradox, as the items used to save himself would not exist.
So for example, they could go back and save Sally's friend, but only after the point that she wrote the letter and gave instructions how to deliver it.