Kurokawa Onsen does not do single-bath tourism — the whole town is built around a wooden pass (nyuto tegata) that gets you into three different ryokan baths of your choosing, for about the price of one hotel’s day rate. Buy the pass at the tourist information center near the river, not at the ryokans themselves.
Prioritize the outdoor rotenburo with river or forest views. Several ryokans face directly onto the Tanoharagawa, and the walk between baths along the river path is part of the experience, not just transit. Go on a weekday if timing allows; weekend afternoons mean waiting outside the popular baths.
One practical note: the pass is valid for six months from purchase, so it is a reasonable buy even if you are not doing all three baths in a single day.