Iron in the blood is paramagnetic. It is not affected by magnets.
I was thinking maybe one inside you would react to outside metal like say keys or a belt or a metal chair/desk
The amount of iron in your blood is really minuscule, I don't think there's any risk there.
What I'm more interested in, actually, is what happens if you swallow just one? Are these magnets strong enough to influence / interfere with the iron in your blood?
Strong magnets are REALLY cool, and REALLY fun. The closest damn thing to magic, in my opinion. I think I'm going to go buy some of these balls before they get banned...