He cycles through buffs, each of which reduces the combat effectiveness of all nearby members of a specific race. Each Aura lasts 60 seconds. So all you need to do is wait till he starts a new buff that is NOT your race, then pull.
Of the two of them, the northern mob is easier to get to. The southern one has a lot more mobs to fight through to get to him.
To get to the northern Blind-strider, turn north after swiming past the Bridge-shard and look for a ramp around the waterfalls. Slip by the mobs and turn east at the stream that feeds the waterfalls. From their is a straight shot and you only need to clear a handfull of mobs to reach your target.
Locations with maps: Foundations of Stone