Why do metal stirrups turn your boots black?

The stirrups are likely either rubbing on the boot causing scuff marks or they could be silver plated and the tarnish is rubbing off onto the boot. You should give the stirrups a thorough cleaning, and if they are silver polish them with a silver polish. The stirrup may also be too small in which case they will continue to rub even when clean. Remember a stirrup should be a full half inch wider than your booted foot to prevent rubs and getting caught up in it.