Blood Oath combines elements from hidden object games and point-and-click adventures, where the player assumes the role of a newly-turned vampire that awakes on a remote village without memories. As the protagonist explores the gloomy surroundings of the deserted place, vampires and other characters start to appear and explain the current situation, trying to persuade the novice vampire to choose alliances and make some careful moral decisions.
The main objective is to find a series of items to solve all the puzzles and tasks found on every location. Most scenes have special objects that act like hotspots, pulsating in bright red when the cursor is hovering over them. After clicking on one of these items, a new tab with the object name is added to the bottom interface. Each tab displays the silhouettes of all the items that must be retrieved to activate, or perform an action on the hotspot. Often these items are not in the same scene as their related hotspot, forcing the player to go back and forth visiting all the available locations for the missing paraphernalia. As a visual aid, gems encrusted on top of each tab and silhouette glow when the required items are visible on the screen. The hint button is a blood vial at the bottom-right corner that highlights the position of one of the required objects, or circles a location on the map with pending puzzles when activated, taking some time to refill after use.
Most scenes have small dominoes engraved with special symbols scattered around the scenery, that can be collected and used on a mini-game to increase the efficiency of certain help functions. They have to placed on a checkered board and aligned to create chains of the same symbol and increase the level of each hint option. The ankhs make the hint button recharge faster, the blood drops multiply the number of points received, the moons make the powers activate faster and the skulls reduce the time penalty for wrong clicks. The vampire powers are received on pre-determined points in the game, essentially giving access to previously barred locations, with the main character becoming a flying bat, transforming into mist, and gaining other similar abilities to reach certain places.