I reckon Phineas must've heard it from Sirius's parents! perhaps Narcissa/Bellatrix/Andromeda's parents would come round and discuss dark things. I wonder if it's possible he has other portraits as well... this is giving me ideas for my fic. We only know of him having two, but he could hang out in other dodgy people's homes, eavesdropping and picking up their lingo.

You're not keen on Dark Lord, but how about Lord Voldemort? in a world that seemingly doesn't have the peerage?