64 Frost St., Brooklyn (718-384-8902) The Black River, a cocktail at Battery Harris, in Williamsburg, is so well advertised (“rum, cinnamon syrup, splash of Coke, Angostura bitters, lime”) and so sexily served (in a rocks glass, with a cinnamonstick swizzle) that you might not realize, until you take a sip, that it’s a dressed-up rum-and-Coke. This works as a metaphor for the bar itself: you might not realize, until you explore the expansive back patio, just how close it is to the B.Q.E. (It used to be a gas station.) Inside, the vibe is beachy, with tiki furniture, thatched ceiling fans, and a dollar-a-game pool table; the owners, who grew up in the Rockaways, named it for a bunker at Fort Tilden. The kitchen serves excellent Jamaican-Latin fare, including flaky…