Ahh I see, I think we didn't travel far enough. Cheers for the info.
The ruin is right on the path, but if you're near a waterfall, then chances are you haven't gone high enough up Blackhill, and the brow of the hill is in the way. As you go up the path on the east side of Olchon Brook, it eventually starts to level out on top of the mountain, and the ruin is up there.