carsten@kragelund.me writes:

> Fontifying src-blocks with a missing language would allow being able to 
> theme src-blocks without having the unthemed text of languages fallback 
> to whatever color the src-blocks are set to.
> I envision this could be done with an empty string in 
> org-src-block-faces.

I do not see much problem adding this.
See the attached patch.