If it was down to numbers on the brain it's lucky WoK wasn't being made by the later Trek producers then, or Khan would have been down there for 47 years...
As far as V'Ger goes, though the intent of the "What they used to call a black hole" is presumably that they don't call them that by Decker's time (not entirely an unreasonable guess as it was, IIRC, still a fairly newish term at the time), I personally like to think he just means the specific thing the probe fell into was something humans at one point thought was a black hole, but actually turned out to be something else entirely and was subsequently renamed (the "Black Star" theory mentioned above would actually fit this pretty well). The same way someone might describe Pluto as "What they used to call a planet".
As with most continuity fixes, there's a bit of fudging there but is manages to explain both its strange flinging-things-across-the-Universe (rather than, say, crushing them to death) behaviour and the fact black hole continues to be used as a name for, well, black holes in all subsequent Trek.
EDIT: Though thinking about it, Decker does seem to have a funny thing for strange phrases no one else uses. I think he's the only person to ever say "Star Hour" as well, perhaps it's just the case he's stopped calling them black holes and everyone ignores him no matter how much he insists his name of "SPACE hole" is better?