Keep blacks beans and rice warm.

This recipe should definitely work if you follow it correctly. Comparing your recipe to it might tell you what you did wrong. From the information you already gave, it seems certain that you didn't cook it long enough (as rumtscho said in the comments). Other things that could have been wrong:

• oven not hot enough (your oven might not be as hot as you think)
• water not hot enough when added to the rice (Alton Brown's recipe calls for it to be boiling; I don't know about yours)
• too much water/too little rice
• dish too small (if the water is deep, it won't heat as fast in the oven)
• (thanks to Aaronnut, see the comments) non-metal casserole dishes take a while to heat up

Given your updates, well, basically I have no idea how that recipe would ever have worked. As you saw, the water wasn't even hot enough yet after half an hour. If you poured the water in boiling, it might have had a fighting chance, but it still would probably not have been long enough.

Cooking it uncovered is weird, too - it means that once it's hot, you'll start losing a lot of water to evaporation. The recipe on the bag likely has you cook it covered on the stove, so clearly you shouldn't be using the same amount of water with it uncovered.

As for the amount of water... 1:2.5 really sounds like too much water to me. I've seen some people online claiming that 1:2 is right for white rice, and brown rice needs more water, but in my experience that's a recipe for soggy, watery rice. (Edit: Of course I was basing that on covered rice; rumtscho rightly points out in the comments that for uncovered rice you'd need even more, since you lose some as I mentioned.)

So if I were you, I'd forget about the recipe you used, and try the one I linked to. Many people (including me) have had reliable success with it.

It sounds like you were expecting slow cooking to be like sous vide. Well, it's not. The point isn't controlled sub-boiling temperatures, it's something on the border between simmering and boiling for foods that just need a long time to cook at that approximate temperature.

Slow cooker recipes are not supposed to be very sensitive. They're expecting to be approximately boiling for most of the time, and the difference between low and high is pretty much whether the boil is marginal or a bit more substantial. In many cases, this just matters because the quantity in the pot varies, and it takes more to keep the stuff at the top hotter if it's farther from the bottom, or if it hasn't all cooked down into the liquid yet. In either case, it shouldn't be a full rolling boil; it's just boiling on the bottom, so the rest of the liquid is probably a bit below the boiling point. And yes, this is still slow cooking. It's not boiling fast enough to lose a huge amount of liquid (or worse, boil over) with the lid on.

I wouldn't really try to assign temperatures to slow cooker recipes. Like I said, they shouldn't be that sensitive. If your bean soup recipe didn't work, maybe it was a bad one. But "reliable" in the context of slow cooker recipes doesn't mean "exact times and temperatures". They're generally things that will be perfectly fine if you cook them 25% longer. Not everything in the kitchen has to be precise and formulaic; slow cookers and slow cooker recipes take advantage of that fact. And even if you do try to calibrate, you'll have trouble, since there's a temperature gradient from bottom to top. Unlike sous vide, a slow cooker is not constantly well-mixed. In equilibrium it'll be boiling at the bottom, and 10-20 degrees cooler at the top.

If you really wanted to use a sous vide controller, I imagine something like 95-98C would work for basically every slow cooker recipe, no matter whether they say high or low. Of course, mixing thoroughly enough to make your controller actually work, you may be overstirring whatever you're cooking. But the point is, things you cook in a slow cooker aren't really going to care much what the exact temperature is; it just matters that it's hot, near boiling, and not boiling so fast that it sticks on the bottom or loses a lot of liquid.

Finding reliable recipes... Well, it's like anything else. If you're looking on the internet, you have to learn to judge for yourself and look for warning signs, or stick to sites with lots of reviews. You also have to accept that sometimes you have to test for doneness and be flexible about time. This isn't really unusual; baking recipes should always have some kind of test ("until golden brown") and the actual baking times will vary. (With something like bean soup, sure, maybe the recipe was bad, maybe you didn't soak enough, maybe the beans were a little different. A stovetop recipe wouldn't have been precise either.) If all that isn't good enough for you, buy a slow cooker cookbook; tons of those have been published in recent years.