Will the bad grade in a proposed studied subject be a red flag in my application?
I don't believe there is a serious stigma associated with an undergraduate getting a B+ in a graduate-level course. It is generally understood that undergraduates taking true graduate-level work (with graduate-level course numbers, etc.) are taking exceptionally advanced subjects, relative to their own preparation. Therefore, it is entirely understandable that you might have an A-, B+, or even a B in a graduate-level subject without it having a strong impact on your chances for graduate-school admissions. Exceptionally weak performance (corresponding to "failing" grades—usually C+ or lower) would be an exception to this, but I see no reason here why you should worry too much.