Nobody can tell you anything meaningful just based on the fact that you got a B+ in a single class. Grad schools look at all your grades, your GRE scores, what university you're at, your research experience, etc. etc. etc. So stop freaking out.
(Tip: talk to professors who are doing your preferred kind of research at the grad schools you want to apply to, and see if you can convince them that they want you to come there.)
You don't need to have a perfect average to get into a good graduate program. To get into the popular ones you need to be ultra-competative, but if I understand correctly and your lowest mark is a B+, you should be fine. You can bolster your competativeness with some research and/or teaching experience.
Yah that's silly to think a B+ is going to ruin you. This isn't like high school where the best college programs are bombarded with a thousand applicants all with 4.0's and some SAT score. When you apply to graduate school, you have a lot more "tools" at your disposal, GPA, GRE recommendations, research experience, publishing papers, giving talks... etc etc.
One B+ isn't going to ruin your grad school chances. That said, we know nothing about the rest of your application (GREs GPA, research, etc.) so we can't really comment on what your chances are like to begin with.
Isn't that like a 80% score? (not used to letter grading)
Seems unreal to me to have to get an A all the time...?
I see so many people that post they ''only'' get B+ etc, but I have an average grade of 80% and I am amongst one of the highest. Is getting a B average considered ''bad'' in America ? It amazes me.