Iron is magnetic because when it is clumped together, the electrons connecting the the material move much more freely then in combination with other things like carbon (including iron mixed with other things)
When magnet gets close to iron, some of the electrons can alight themselves with the magnet and make the iron magnitized. You can even make the iron stay magnetic afterwards.
However this is a property of a CLUMP of PURE iron. Bond the iron with something else or try it with a single atom of iron and this won’t happen.
Think of it like water. In a group it’s a liquid and can hold objects up. On its own it’s just dust.
The iron in your blood exists as individual ions which are then bound to proteins. These ions are restricted in movement and cannot easily form magnetic domains, where each iron atom in a domain is “pointing” in the same direction. This is a pre-requisite for magnetic properties.
Since the iron in your blood cannot from magnetic domains, it cannot demonstrate significant magnetic properties and will not be attracted out of your body by the MRI.