If I were to be possessed by the urge to treat the boot rubber, I'd reach for one of those water-based silicone-ey "space-age" protectant substances whose names I've forgotten because I so rarely use them for anything except the door rubber.
You know....the ones that morons rub on their car interiors right before they sell the car, forcing the new owner to spends hours cleaning the creepy, slimy, migrating stuff off?
Seriously, unless the boots are coated with an stuck-on abrasive or glass shards that might cause them to self-erode, you might be better of leaving the black dust buildup in place as a protective coating.
Same reason it's better to leave the faded outer layer on your tires...it acts as a sacrificial first layer to environmental contaminants.