I’m not too familiar with Blink products, just that a wall wart bell showed up in a quick search, so it is possible. Most camera doorbells I’ve seen that do have a pair of terminals, they’re to do as you described: connect to the bell circuit to get power and ring the existing bell. I’m doing a bit of inferring, since knocking sounded like the only previous option, that there’s no wire at their door to utilize. Couple that with their being in an apartment and it is likely the only way they’re getting one there is to surface run it. Or run behind any crown molding/baseboard trim they have, but that’s a labor-intensive pain in the ass.