It seems as though White can easily dominate the tempo in a blitz game through a strong, aggressive opening. Is this the case? What openings are strong for White when there is a short time limit (e.g., in blitz)?
I think this depends very much on how strong you are.
That said, I had some success with the Ponziani. It is not the "best" opening, but solid, and many players don't know it and lose some time thinking about the next moves. The theory is easy, and there are several traps black can fall into (my favorite is when black takes the "unprotected" e4 pawn with the knight and you can fork it with
Generally it might be a good idea to play aggressive (e.g. Steinitz'
Qh4 in the Scotch game) and to try to throw the opponent out of the book as soon as possible. Of course the opening must be still playable, and you need to know some theory.
There are plenty of openings you could use in blitz, but I think the slow solid and positional openings need more thinking, thus you could lose. I would suggest playing like Mikhail Tal. That is, make sacrifices without calculating the move too much, and try to complicate the position for your opponent.
I have used this in many of my games. Complicating the position by making a complex, unsound sacrifice may confuse the opponent and make them lose on time or make a wrong move. Remember, clock is also a thing you will need to think about in blitz.
I'd suggest playing the king's pawn opening as White. Maybe the king's gambit, the open Sicilian, which is my favourite, Scandinavian, and such. French is a solid opening from black and either play advance and lose time for your opponent or play the exchange variation.
With the black pieces, against 1.c4 Nf6 is the best way to respond. After that try to enter the Indian system, Nimzo, or King's Indian Defense.
For d4, I would say Nf6 isn't bad but it involves more thinking, so I like to play the Dutch defence, the Leningrad variation (which is not a very popular a chess opening).
I would definitely suggest the Najdorf variation in Sicilian, though it seems very impractical because of the sharp lines and tactical, calculative play, as mentioned before you can confuse your opponent into time pressure. Of course, I recommend making unsound sacrifices wisely, when absolutely necessary. Remember, even if your playing online you are psychologically twisting your opponent into a very uneasy place.
You should analyze your results at blitz versus bullet before reaching any conclusions.
Here's a notebook you can use for that purpose if you are on chess.com