[SamBam]

I wonder if deliberately sending money to a blockchain address that no one has the keys to could be legally considered destruction of money?

Probably not yet, but maybe eventually. It's essentially the same as burning it.

[yathern]

Destruction of value is not destruction of money. If I were to buy some extremely expensive memorabilia (a pristine copy of Abbey Road, signed by all of the Beatles) and burn it in my back yard, that wouldn't be destruction of money.

While BTC, ETH, XMR function like currencies and can be traded - they are not recognized as a currency in most places