That sand may be more than merely magnetic - mineral sands can be noticeably more radioactive than the average background radiation level. Generally, it's nothing to worry about, though.
I assume your black sand came from weathered basalt. Some of the minerals in basalt are potassium compounds, and in all naturally-occurring potassium there's a tiny percentage of a radioactive potassium isotope. Potassium is essential for all living things (AFAIK), so we all have a little bit of radioactive potassium in us. Some plants are quite high in potassium, and so they too be can more radioactive than the average background radiation level. See http://en.wikipedia.org/wiki/Banana_equivalent_dose
As well as the potassium, it's not unusual for basalt to contain heavy radioactive metals, like thorium, uranium and radium. These elements are certainly not
desirable trace elements! But most of their isotopes have long half-lives so the radiation they emit is of low energy, and so in the very small concentrations found in unprocessed minerals these elements usually
pose no danger.
OTOH, I wouldn't want to breathe fine dry mineral sand dust on a regular basis, but that's because such dust is bad for your lungs even if it's not radioactive. Also, houses built with igneous rocks like basalt and granite can be bad for the health if poorly ventilated. Some of those heavy radioactive metals emit radioactive radon gas as they decay, and if that gas is allowed to accumulate it can be dangerous to breathe, as it's a leading cause of lung cancer. See http://www.cancer.gov/cancerTopics/factsheet/Risk/radon