Would a B grade PhD-level course in part time help or hurt graduate application?
Hiding the grade is unethical and if discovered will haunt you. But lots of doctoral courses in the US, at least, result in B grades. It isn't a huge problem if you learn something in them. Their purpose is to prepare you for comps, not to show a high GPA. 

I'd call in more or less neutral for your application. But the application, overall, needs to show both preparation and likelihood of success. I don't think a B in a difficult course is a negative for either. But the application overall has to demonstrate excellence. It needn't be perfect in every detail. 

For students already in the program, a B grade is considered acceptable. 

But, if you restrict your search to "top 5" graduate schools you are already likely to miss the mark due completely to the small number of slots and the large number of highly qualified applicants.