Bar Blanc, Allegretti, or Scarpetta?
Of the restaurants you're considering, I've only been to Allegretti. We've been there three time and have become huge fans. The Provencal cuisine is superb; service is pleasant and professional; the decor is warm and attractive; the noise level is conversation-friendly; and the overall ambiance is adult and civilized. I think Allegretti is a perfect place to enjoy a celebratory dinner.
I have not yet been to Allegretti, but have had many superb meals at both Bar Blanc and Scarpetta. I think any of the three would be fine choices (Scarpetta will be tough to reserve at this point), depending more on your personal food preferences and which menu and ambience appeal more. Bar Blanc and Scarpetta are more trendy, though I have not found either to be excessively noisy. At the same level of food quality and price, I would also recommend Olana, which is extremely comfortable, with big chairs and well-spaced tables and more formal service. Convivio is another good option, though it is a bit above 34th St.