This place came highly recommended by a hot spring master in Beppu, so I set out to find it on a return visit.
This onsen is definitely a local secret - it's not mentioned in any guidebook, you can't find it on any tourist map, and the entrance isn't too well marked.
However, it's not difficult to find, as it is located in one of the big touristy areas of Beppu - the Kannawa jigoku tour. The jigoku (地獄, loosely translated as "hell") are hot spring areas with unique features. These hot springs are for viewing, not for bathing.
The bathhouse is on the road before you reach the main inn. Just walk right up to the inn's threshold and get someone's attention. If you're loose with Japanese, try saying "onsen dake" (only the onsen). It costs ¥800 for adults and ¥400 for children. There is no time limit, but the ryokan guests have priority; that is, the bath is only available between 10:00 and 4:30, between check out and check in.
As for the onsen itself... very relaxing. This is an uncrowded, well-maintained, seldom used onsen. The dressing area would be a tight squeeze for a dozen people, and is very modern, featuring a vibrating foot massager and everything you need to change and wash.
Step down a flight of stone stairs and you're sure to enjoy the first stage - the showers and the hottest bath. Take your time in the indoor bath because it is significantly hotter than the others.
Next up - the main course. The rotemburo (outdoor bath) at Kannawaen is very unique due to it's milky blue water rich in silicon. This water changes color from dark to light blue depending on the mineral content. It's warm, the area around the bath gives you the feeling you're surrounded by nature, and the water itself is quite healthy. Stay in as long as you want, and don't rinse off so as to take advantage of the mineral properties on your skin.
In one of the more well-known areas, having a secluded feeling, and unique in appearance and quality, this onsen is a fine attention to Beppu. Ikimashou.
Kurokawa is one of the best onsen villages in Japan, and one I have not yet had the pleasure of visiting. Located northeast of Kumamoto, it is not accessible by train, and it would be best if you had a car.
Set in the Japanese countryside, these onsen are tucked away from the outside world and in a more traditional Japanese setting. One of the best baths there is Shimmeikan, which features cave baths and huge rotemburo.
If you're looking to try different onsen in the area, the tourist information desk provides you with a wooden medallion for ¥1200 that gives you access to three onsen of your choice.
Although I highly recommend staying in a traditional onsen ryokan at least once (around ¥12,000-20,000/night), you have the option of commuting from the cheap Aso Senomoto Youth Hostel (¥2300/night) and just visiting the baths.