a ferrous choke is a tube magnet, it filters out noise on the circuit, by sending it trough a magnetic field, doing so only the strongest frequency's make it trough the tube's field, the weaker one's bleed out as they cycle trough the field.
it could be that the current of the resistor was drawn by the magnetic field, and created an arc, anyway you should make the inside of the block dry with a blowdryer and seal it with tape and keep that magnet away from everything, there is a reason there is a plastic case around it, apparently it is not thick enough.
and if the screen works there is no damage done..