That is actually very much sounding like a crack in your screen. That black spot you describes is exactly what happens to a cracked LCD screen. It may be small or look like a scratch or even like a hair on the screen, but it sounds like a cracked screen.
As for should they take it back.... I would say no. If it is a broken screen it sounds like you would be responsible for that. it could be small enough that you could sneak it past someone live, but if vzw sends you a replacement through the mail and you have said there is no damage, and then they find damage as they take the phone apart, they then could charge you for the phone.
Good luck. It probably was some pressure on your phone that you didn't notice just over stressed the screen.