Blood Gracht is only available in Guerrilla Warfare matches. You can set Blood Gracht as your preferred map for Guerrilla Warfare in the Main Multiplayer menu. Your will need to achieve 5 kills without dying in this map. This map is very small so there plenty of chances to get cheap kills with grenades and Proximity Mines.
If you're new to the map, play it for a couple of rounds and you'll get a hang of where the common firefights occur and you'll learn a trick or two from the in-game players. You can also get some cheap kills leaving multiple proximity mines all over the map and playing as the Marksman, abusing the Cloak ability. Having a Medic with the Triage+ ability around will also save you from bleeding out if you're mortally wounded, retaining your killstreak at the same time. You are awarded 100 points for achieving a 5 kill streak.