Do you know that if you have
Darkest Hour in play, you can't activate the first ability of
Teysa, Orzhov Scion? This is because all the creatures are black, only black, not black and the other colours they have. So you would never have 3 white creatures to sacrifice.
On the other hand, like all creatures are black, even if you can't sacrifice creatures with Teysa, if you sacrifice them with other cards, you always will recive a new creature to remplace it becasue the token that Teysa creates is black too.