Too late to add…
I think your guidance counselor comment about one B+ grade is not going to matter…at all. Better for the school counselor to concentrate their letter on your strengths and other applicable things…explaining away ONE B+ grade is going to look silly.
Your plan to take some additional history courses is not a bad one.
Do keep in mind that this one B+ grade is going to have zero impact on your college admissions so when the time comes, if you do get rejected from any colleges…don’t blame it on one B+ grade.
With regard to explaining…it would be one thing if your whole term marks in every subject were affected…but they weren’t.