To be honest, I'd avoid getting an all black cavalier from 99.99999% of 'breeders' because no reputable, health focused breeder breeds for these colours, and the backyard breeders who do breed them, tend to charge a lot claiming these colours are somehow special. They aren't.
I am just wondering if the breeder was selling these dogs citing their special colouring as a GOOD thing, or is she a reputable show breeder selling them on a strict spay/neuter clause to a pet home because this is an unexpected recessive trait that is considered undesireable to reproduce? I have seen the websites of a couple of people who breed, deliberately, for black cavaliers and sadly, none of them show, follow the very important
MVD breeding protocol, test eyes, hips, patellas. MRI screen for syringomyelia... indeed some even advertise especially small black cavaliers which is deliberately breeding with no regard for health because miniaturisation means someone is breeding runts and substandard dogs. Right now, breeding for extra small dogs is already known to predispose them to smaller skulls which in turn is probably predisposing for the potentially severely painful condition
syringomyelia.
I'd be very careful about taking a puppy from anyone who isn;t following strict health guidelines and isn't a show breeder who actually understands genetics and the serious health issues that are so easily introduced into this breed. If the breeder fits
all the criteria for an excellent breeder and this is just a pup that is unexpectedly black, that is of course a different situation.