If it's something thats done incredibly frequently, like updating information on other characters on screen, even a tiny change could have a non-trivial impact. It also doesn't really prevent people on figuring out alts, it just shifts the effort of doing it around. A malicious person can stand in limsa, grab some network data, add "Char X" to their blacklist then re-grab the data and they'll see "Char Y" no longer in the response. It'd probably be an improvement over the current one, but the tradeoff of added computation/latency on every single update from the server may not be worth it. I don't think there's a way to have account-wide blacklisting that doesn't inherently have some degree of can-find-alts-if-dedicated-enough problems.