The Blood Gem Contact trophy is awarded for finding your first Blood Gem, which is an item you can fuse with a weapon to increase its capabilities. The first Blood Gem you'll probably find is located in the basement of a cathedral near the Cathedral Ward lamp.
From the lamp, take the left hand exit of the building. Pass through the courtyard, dealing with the staff-wielding hunters if need be. Proceed straight ahead, past a terrace looking out over the city and proceed down a flight of steps to your left. You'll emerge in a plaza surrounding the cathedral. There will a few enemies patrolling by a nearby bonfire; clear them out if they attack.
The cathedral entrance is open, but you won't be able to get to the basement just yet. Proceed outside to the left of the cathedral and up a flight of steps. Watch out for the sniper at the top and his pair of dogs. Once inside on the upper floor of the cathedral, take a right through the doorway and you'll find yourself on a balcony, looking down into the main cathedral chamber. Pulling the lever on the balcony will move the sarcophagus down below, exposing the stairs down into the basement.
Down the steps, you'll find a chamber full of statues. You'll find the Tempering Blood Gemstone ahead, in the corner. Congratulations!