Killing off the black lesbian is 100% something Steven Moffat would not do and you’ll see that on Saturday.
I don’t know if anyone’s up for some HUGE SPOILERS!!! but-
Of course, that then shifts the conversation out of “it’s really not okay to kill the first black lesbian on your children’s show” territory and into “Is it really okay to leave fans of the first black lesbian on your children’s show hanging for a week about whether she lives or dies” territory. (And I’d say ‘no’ to that last questions, honestly.)