I'd say you could still call mutable blockchains blockchains, because that's still what they're trying to emulate anyway. They'd still be closer in both function and form to run-of-the-mill blockchains than traditional databases. That being said, I don't see the point, and I wouldn't be surprised if they ceased to exist in the future. There was a CW piece that tackled this a while back:
Within a private blockchain there is also no ‘race’; there’s no incentive to use more power or discover blocks faster than competitors. This means that many in-house blockchain solutions will be nothing more than cumbersome databases.
Another reason why I'm fine with calling them blockchains is because language is inherently fluid anyway; words change depending on their usage. For as long as people call them blockchains (and they do), the definition of the term will be adjusted to accommodate them.