So I found the key to meet her, which is her daughters blood on a handkerchief, and I guess I just assumed I'd use it then and there but I guess that's not how it works. Could anyone steer me in the direction of what to do next?

Well, a key is only useful if you find the portal it goes with. See if you have any leads on where the portal might be; for that matter, you might want to do this for anything you left behind in a prior incarnation. You should do fine without spoilers, but let me know if it's not enough.