B+ in graduate level class related to area of research

[gravwave]

I am an undergraduate and took a graduate level class in my area of research. I received a B+ in this class. Normally I would not worry too much, as I know one B+ doesn't make a huge difference, except that it's related to my research field (I plan to do experimental gravitational waves and the class is graduate GR). How will this affect my chances of getting into top graduate programs? I have significant research experience in this field and great grades otherwise.