Should I mention this health issue I had on my graduate school application?
I think it is worth mentioning.  Of course, you don't need to say "my health issues caused a B+".  Just mention the health issue at that particular time, and what you had to do to complete your work.  

Besides explaining your performance that semester (and a single B+ isn't very much to worry about), this shows your ability to overcome adversity and still succeed.  

Here are some other questions about explaining lower grades for non-academic reasons, and the answers largely seem to agree that it is a good idea to mention the circumstances:

Will one &#39;C&#39; grade due to health issues ruin my chances of admission into a top grad school?

Trauma affected my grades while in undergrad, how will it affect me getting into graduate school?