German is my first language and I, too, found this sentence to be a little odd (then again, most are ...). It is grammatically correct, but really only makes sense if you were talking about her before. "Es ist eine schwarze Maus" would be completely acceptable when talking about that thing that just scurried through the room.
The reason why the mouse is "sie" (or a she) is because the gender of the word is female. Germans often do this with objects as well as living things. E.g.
Q. "Ist der Tisch alt?" A. "Ja, er ist alt"
Q. "Ist das Licht kaput?" A. "Ja, es ist kaput"
Q. "Ist die Maus tot?" A. "Ja, sie ist tot"
A masculine word can be referred to as "er", feminine "sie" and neutral "es". This is just because the gender exists in German.
For native german speakers: 1) if you would hear someone saying "Sie ist eine schwartze Maus" or "Sie isst eine schwartze Maus", could you tell the difference without any context? 2) if you were in the situation of saying either of the two sentences, knowing the ambiguity, would you have a way of "inflecting" the pronounciatin (without changing the sentence) so that your interlocutor would get the right meaning? Thanks!
I'm quite sure that if a statement like that was made. Sie isst eine schwartze Maus. It be preceeded by an Echt! Sie isst eine schwarze Maus. and said slowly and softly as in disbelief. Where as a woman leaping up on the kichen counter would be saying *Sie ist eine schwartze Maus! Loud and fast as tho startled. Words alone are a very poor means of communication. When people interact, more than 60 -70 percent of our communication is done through vocal tones and inflections, hand and body gestures, and facial expressions. People speak volumes about themselves just by the way they walk into a room full of people. For good or for bad, and most don't even realize they're doing it. But, when you do open your mouth, having a solid vocabulary, good grammar, and proper social skills go along way. But you do need both, one without the other sends mixed signals. And we've all seen what happens to those people. Nothing good
has determiner, nominative (sein never takes akk), feminine. m/n/f/pl: e/e/e/en https://www.lsa.umich.edu/german/hmr/Grammatik/Adjektive/Adjektivendungen.html