If you buy only one new piece for fall, make it a blazer. A blazer is that magic piece that will instantly pull together any outfit this fall and winter. Not only does it come in a multitude of styles (think: tweed, wool, cropped, and double breasted), its transitional power from work-to-weekend is undeniable. For the office, a date night, or casual weekend festivities, here are 15 fresh ways to style yours: For the office: Atlantic PacificGo for plaid this season, instead of classic navy…