Lukewarm grade in proposed area of study? I'm applying for study in a particular area of math.

To keep matters short , I was an undergraduate that took the graduate version of a particular class and ended up with a B+ . I had never taken any class on the topic before this.

The department received several complaints about the professor in the course and other professors in the department even acknowledged that the distribution of the grades was quite terrible , not expected for a graduate course. 

Since the grade is in a class that is in my proposed area of study , should I mention it at all in PhD admissions? My supervisor and other professors in the department said they would mention that the class in question was terrible but I'm not sure if I should  talk about it myself.

I feel kind of strange not addressing the fact that my proposed area is the only course which I didn't do great in. For some context every other grade I've ever received are great so there is no pattern of doing bad or anything.
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.