What opening should I learn as black?

@nolocals Playing 1.Nf3 can rule out certain options for Black but the game usually transposes into some 1.d4 line. I'd recommend playing 1.e4 over 1.d4 or 1.Nf3, to get familiar with open positions, sharpen your tactical awareness.

And same thing goes for French Defense. After the moves 1.e4 e6 2.d4 d5 3.e5 c4 Black has to work around their poor space control, a bishop on c8 that they can't develop. You can learn the French opening theory and get a good position against people with your big pressure on d4 but you might later realize that these strategies are rather specific to French and you're used to playing in a particular pawn structure.

Playing e5 or c5 against 1.e4 prevents white from putting a pawn on d4. Resulting pawn structures from these openings can be vastly different so you can enjoy and practice the different aspects of chess.

@EdinburghCollective so Sicilian defense vs. e4 and a d5 opening against d4 is a good place to start?

Yeah, c5 is the most popular and probably the strongest response to e4. I'd still suggest e5 over c5 because c5 can be hard to learn, but then again why not start learning right away? You can switch back to e5 if you feel like white has too many different responses. And d5 is a good response to d4 at any level. You can learn some Slav Defense or Queen's Gambit Declined lines.

If you find d5 a bit dull and samey, Nimzo-Indian would be a nice alternative (again, viable at any level). Easy to learn the the basic position. But from that position you can apply different strategies and go for different pawn structures. And even choose employ the hyper-modern ideas you seem to talk about fondly.

Why not the CK? It is a bunch of those things most players are searching for:

By the way, Matthias Wahls is a GM!

http://matthias-wahls.com/67-5-reasons-play-caro-kann/

At your level the best opening is the simplest opening. With white it is 1. e4. With black e5 or d5, depending on what white plays. If 1. e4 e5 then often d6 is OK, if 1. d4 d5 then usually e6. Learn fool's mate and scholar's mate. It can be useful to learn a simple line against the patzer opening 1. e4 e5 2. Qh5.

I think Caro-Kann is the most sensible option among the openings where you play a fast d5 against e4, others being French and Scandinavian. In French you block in your light square bishop and in Scandinavian you make a bunch silly of Queen moves.

Caro-Kann seems like an "easy to learn but difficult to master" type of opening. But I suppose chess is like that in general. A few basic ideas can get the beginner going but if an opponent is familiar with it beginners can get into trouble. You have to be careful of a few nasty king side attacks and the potential of getting positionally cramped (but not as bad as the French). Otherwise, it's pretty solid and doesn't require too much theory.

If you wish to reach similar pawn structures, play related systems ie: {Collie System, Caro Khan, Slav} or { KIA, KID and modern/Pirc}

@clutchnutz Chess is not an easy game to learn. I remember myself sometimes making illegal moves (or not noticing my opponent making it in OTB games) even after 2 years of playing. I've seen here players 1700 rated who don't know how to mate with a rook vs king. I could go on and on.

@pointlesswindows Chess is a very easy game to learn. It's not uncommon for kids to learn how to play before they can read or do basic math. But learning how to play a game is very different from learning various openings, endings and tactics. That's why I said it was difficult to master.