I'm looking for a sharp and strong opening for black, ideally for 5 minute blitz.
My absolute favorite for white is fried liver (1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4. Ng5), which leads to equal development and a couple of pawns with best play, and a rook or quick mate with worst play. Is there some similarly sharp option for black?
1. f3 e6 2. g4 Qh4#
Pretty sharp
But seriously, look up the Budapest gambit ;)
Scandinavian Gambit, if it's for Blitz. Makes for some surprises ...
1. e4 d5 2. exd5 Nf6 3. c4 c6
If the opponent is sensitive, mix in the threat of your Big mama
Elephant gambit is undoubtedly the most aggressive and open.
Although it's not the most sound, if white plays carefully.
Try the Spanish Game, Berlin Defense, Fishing Pole Variation:
1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 4. O-O Ng4?! 5. h3 h5
Although it is not sound against best play, it can wake up an unprepared, sleeping opponent fast! Most gambits are usually unsound against best play, but then again it is usually a lot easier to attack than defend.
It is most meet we arm us 'gainst the foe;
For peace itself should not so dull a kingdom,
Though war nor no known quarrel were in question,
But that defences, musters, preparations,
Should be maintain'd, assembled and collected,
As were a war in expectation.
W. Shakespeare -- linebacker for FSU
The fishing pole rarely works in my experience, but it is worth a try; the rejection of the fishing pole is still playable. It is a relatively safe try for black, even if white doesn't bite.
Spanish Game - Four Knights Rubinstein Counter-gambit is pretty solid as well if you're already playing for Fishing Pole and White surprises you with a Four Knights instead.