I'm wondering what has caused this 'smut'. It could be the sign of an infestation of some kind. I think you need to find the cause of it. As far as removing it, have you tried a hose? It would be a tedious job to wipe each leaf with damp paper, unless it's only a small specimen.

If by 'smut' you mean a black dusty staining on the upper sides of the leaves, then that sounds like sooty mould which forms on the honeydew exuded by aphids. Have a look at the undersides of the leaves towards the top of the plant, especially against the leaf midrib, and see if there are small pale greenish flattened discs stuck there. If so, that's scale insects, sucking the sap of your holly leaves and pooing out the honeydew as they go. That's what you need to treat. I usually wipe all mine off if the plant's small, but you may need to apply a systemic insecticide. Phil J