If you mistakenly think that a blue plate special is served on turquoise-hued dinnerware, get ye to architectural historian Marc C. Wagner’s lecture and slide presentation “Virginia Diners and Roadside Architecture.” Wagner presumably knows his chrome siding, his vinyl benches, his easy-wipe tabletops, and—tastiest of all—his greasy spoons. Skip dinner; you’ll want someketchup-laden fries and homemade coconut-creme pie after Wagner speaks at 7:30 p.m. at the National Trust for Historic Preservation, 1785 Massachusetts Ave. NW. FREE. (202) 298-1100. (NodB)