Too much risk, perhaps. A Black sister would be suspicious seeing somebody parading around the Tower, proclaiming they're Blacks, especially since they already have ways of communicating that don't require outing yourself. Why would any Black sister wish to expose herself like that and lose the advantage of anonymousness that all others have? You'd have no idea which sister might have recognized the sign and no idea when they were going to contact you or whether they would do it alone or if the whole Heart would come to meet you. And what if somebody saw you that knew you weren't Black? IIRC Talene had no idea that it was only Alviarin who knew the identity of every Black sister.