There are two spawns in each area for the Blades of Crota. The two at Archer's Line both spawn to the left of your spawn with the first in the building closest to the cliff and the second spawning in the larger building where you scan the computer for the first mission on the moon. In Anchor of Light, to get to the first spawn, go towards The Temple of Crota, then where the Captain is turn left and it will be in the cave up the hill. The second is in the room on the bottom floor of the two story building/tower on the far side. There is a vandal with a wire rifle at the top of it. For the Hellmouth, the first is in one of the mobile buildings sunken in the dirt. To find it, head to the cave where the Fallen spawn, turn right and you will see one building covered in dirt. Right past it is an identical building that you can enter and also a possible Blade spawn. The final spawn is in the area directly past the last with a rock wall blocking the section off (it's full of Fallen). There are two ways in. Take the far left path and the first building on your left is the location of the Blade spawn. It will be in the second room in the back of the building.

Earth and moon

Just go patrol, stay in one location for at least 10 minutes - they spawn in all areas