Lukewarm grade in proposed area of study?
A B+ is not a bad grade for an undergrad in a grad course.  I suspect that it will not raise red flags in an admissions process.  The discussion on "complaints" or "the class was terrible" has no place in your application package, especially coming from you, and it would make the applicant come off as a whiner.  You took a course, you got a grade.  Done.    

If admissions slots were limited, and I had to make an otherwise equal choice between  two students, I'd opt for the one that looks more mature, and complaining in your application package about a course does not make one appear mature.