I do think Star Trek: Darkness is better. Kind of goes with the other one word titles, Insurrection, Nemesis, Generations.
That's the problem. "Star Trek: Darkness" sounds like more of the same.
As for Generations
, I often see it spelled "Star Trek Generations" without the colon and I like it that way. It's quite appropriate, given that the movie covers two generations of Trek. "Star Trek Into Darkness", on the other hand, doesn't have the same clever play on words or natural flow to it.