I haven't posted on this board in 3 years, but as a 3L at a T20, I can give you a heads up on what I have seen.
I did 32 OCI interviews right when the economy was going down fast and have seen 2 cycles of this, and I would say that Bs will be a serious problem.
One "B" alone is not a problem, I would say if you pulled all A+s and Bs you would be fine, from what I've gathered from interviewers, is that they care most about GPA, not the particular classes. The people who did super well in Ks and Civ Pro (two things you would actually use very often in biglaw) did not "get one over" on people who did well in Crim Law and Con Law, which are functionally useless in biglaw.
The only grade I have ever heard of being important is legal writing, which I basically failed, yet I got piles of interviews and summer offers.
You need to be in the top 10% to have a good shot at anything in this economy, even in the T20, trust me, I'm there in that situation. Even people in the top 10 are getting shot down in the T20 at T20s in major markets.