The word frijol is Spanish for "bean." A frijol negro, then, is a black bean, and the plural form of that expression is frijoles negros. The Portuguese equivalents are feijão (bean), feijão preto (black bean), and feijões pretos (black beans).
In my feijoada, I used two cans' worth of very soft black beans that I purchased from High Street Market. Your typical feijoada recipe, from what I've read thus far, calls for dried, rock-hard beans to be soaked overnight, which allows them to soften to something that's more chewable but still fibrous. You then drain the pitch-black water and rinse the beans before using them in your recipe. While I'm loving the results of my own feijoada, I think the stew is a bit meat-heavy,* and even though it approximates the color of other feijoadas I've seen online, it's not nearly dark enough: most feijoadas are very dark brown, indeed.
So I thought about buying more frijoles to add to my stew, but because I didn't want to head all the way out to High Street again, I followed my instincts and checked to see whether the local grocery had any frijoles of its own. Korean groceries normally have a gigantic grains-and-beans section; for such a skinny demographic, Koreans love their carbs. Sure enough, the grocery in the building where I work had bags and bags of what appeared to be black beans. I had to be careful because I had seen what looked like black beans in another part of the building, but those turned out to be charred or toasted soybeans—not what I was looking for. In the grocery, the beans looked promising, but they were labeled differently despite looking exactly the same. Two types of black beans caught my eye: one called seoritae (서리태), and another called heuktae (흑태).
To my untrained eye, these beans looked identical, but the heuktae was significantly cheaper. I decided to buy both, and I asked the cashier about the difference between them. I didn't fully understand her answer, but for her, it came down to which beans get used in which food. A bit of online research showed that seoritae were supposed to have some sort of greenish or bluish interior, while the heuktae's interior was supposed to be yellowish. Looking up "서리태 흑태 차이" ("difference between seoritae and heuktae") on Google brought some some interesting results, including this site, which seems to say that seoritae has the stronger taste and is used in "bean curd, soybean cake, and rice" (via Google Translate). Heuktae, meanwhile, has a weaker taste and is used in boiled rice (texture accent) and soy sauce (neutral palette to absorb the sauce's flavor). I guess this explains why it's the cheaper bean.
Either way, whether I'm using seoritae or heuktae, I think I've found a decent substitute for the frijol negro. I have 500 grams of heuktae soaking right now, and as of this morning, it's blackening the water just like the Latin bean would. If it walks like a duck and quacks like a duck... Tonight, I'll be simmering it separately, then bashing the hell out of half of it, then adding the whole thing to my feijoada a little at a time, checking taste the whole way. Tomorrow night, I'll be simmering the completed feijoada all night long in my crock pot. Am expecting miraculous results come Friday morning.
Meanwhile, I have to shop for ingredients for moqueca, that Brazilian seafood stew.
*Carnivores (and Happy Carnaval!) will rightly roar that there's absolutely nothing wrong with meat-heavy anything. Hard to disagree.