1977 S Quarter :)

I have always found it difficult to grade proofs from pictures because slight changes in the angle of the lighting can completely mask the hairlines. Given that you have a modern proof, I think your coin would most likely be PR-67 or above with a deep cameo or cameo at the very least. Most of the modern proofs come in very high grades with deep cameo.

Contrast is the big word. The reason the field looks black on a Deep Cameo coin is because it stands in stark contrast to the blast white cameo'ed design elements. The blackness in photographs is a result of shooting the coin in such a way that the mirrors do not reflect light. It is possible, also, to look at a deep cameo coin with perfect mirrors and see that they are in fact a "mirror". The larger the coin the more obvious this is.

Also, be aware of white collar overpricing with modern coins. I'd say starting in the mid to late seventies the mint had perfected the art of making proofs. Environmental factors aside, nearly all proofs are PF-69 or PF-70 when they leave the mint. PF-70s carry a premium because of a perception that they are perfect- which is not true. PF-70 just means that imperfections are not visible at 5x magnification. There has never been and will never be a perfect coin.