Used to be at harrowstorms you had to worry about the blue souls getting to the witch pikes as they would cause the pike to regen health .. since blackwood dropped, the blue souls don't seem to do anything except just hang out in place ...
They did do something with the HS with blackwood because now you can see which one is up on the map and they always spawn in a clockwise rotation ... before they could spawn randomly at any of the locations ..