She's recurring, and it seems to me she's basically there to finally trigger Erin's run for DA. The thing is, elected District Attorneys are politicians. The current Blue Bloods Manhattan (we have DAs in every borough) DA is a politician. Erin is not. NB: if Erin runs for Manhattan DA, most of her family can't vote for her, as they live in other boroughs.
It's inconvenient for the show to have 5 DAs (as in real life) and they only occasionally mention that people are charged in the different boroughs, but that's how it actually works. Our current DA in Queens is a white woman, the newly-e