It's pretty delicious being blue. You might know Chef Jason Santos by his blue hair. Or you might know him by his two Boston establishments — Abby Lane in the Theater District or Back Bay Harry’s in Bay Village. Or (more likely) you know him from both. The 40-year-old Medford resident’s playful culinary techniques landed him on the seventh season of “Hell’s Kitchen” — he made it pretty far — and he’s set to open his latest endeavor, a Southern restaurant called Buttermilk and Bourbon, later this fall. Santos shares his favorite date night restaurant, where he takes his out-of-town guests and the best little spot in Chinatown.