Think for a moment -- if something is TOTALLY opaque, i.e. 100% opaque -- how do you make more opaque?
That's like asking for complete silence to be quieter -- ain't gonna happen. It can't happen.
Oh, I suppose it could if you happen to have a sound sucker. Good luck finding one.
Oh my God... I just realised I've mistaken opacity with scale...
Even though English is not my first language, there's no real excuse for this...
Anyway, thanks for helping me realise my mistake I need to get some sleep
It works with scale if you apply 3 basic keyframes -- 0%, 120% or whatever, then 100%. And because this is something that normally happens VERY quickly, you shouldn't even need the graph editor. Just highlight the last 2 keyframes, right-click and make them Bezier keyframes.
Oh, you can TRY to use the graph editor, but I think it would be a big waste of time.