Andy N.
Yelp
Chinoike Jigoku (Blood Hell) is one of the many famous hot springs that brings visitors around the world to Beppu. Out of the few core group of hells that attracts visitors, I would say that this is one of two that I'd actually recommend you check out during your stay.
Like the other hells in town, you're not able to bathe in the hot spring. Rather, you are invited to come in and take a look at the blood-colored pond and experience the heat up close but not actually soaking in its waters.
The site was small and for the ¥400 entry fee, I can't really say it was completely worthwhile. There is a small walking path that will take you above the spring for a different view, and a dedicated omiyage shop. If you've been to Yellowstone in the States, it's really going to be hard to compare the Beppu springs with those in Wyoming.
On your Beppu route, I do recommend checking out this hell first before going to the other ones since it's a bit out of the way.