(Part I, Part II, Part III)
In Part III I looked at some general principles of opening study for us 1200-2000 (USCF) players and recommended the repertoire approach, sticking to one opening for awhile, but mainly from White's point of view.
Playing the Black pieces presents somewhat different problems in preparation, but overall I would recommend a similar approach, finding a good repertoire book and staying with the lines for at least 1-2 years until you gain enough experience to get out of the opening in decent shape most of the time. As you become a stronger overall player your tastes and interests may change and your judgment will improve, and eventually you may decide to try out new systems and defenses, but as I said before I think it's wasting your time and effort if you jump around to different openings based on a loss or two. If you're playing a solid, known opening you're losing games because the opponent played better than you in that game!
Some of my own experiences in playing Black and learning various defenses might be instructive (and amusing) to relate. After my first 15-20 tournament games I had acquired several books on the openings lent to me by generous club members, and I started buying them as well. I'd been playing 1. e4 e5 as Black with decent success (considering my strength relative to my opponents) but the book How to Open a Chess Game (by 7 Grandmasters) convinced me to play the French (Portisch was the author of that section).
So I played the French (1. e4 e6) and did very poorly. Knowledge, taste, inclination--none were suited to the closed, cramped nature of the defense. I lost several games (not feeling comfortable the whole time) and abandoned the French pretty much forever (though, strangely I did beat a 1938 player rated over 300 points above me 15 years later when I transposed into the Rubinstein French without even knowing it!). Then a master at the club urged me to play the Sicilian (1. e4 c5) and I have used it on and off ever since, with various forays into the Pirc/Modern, Center Counter and back to 1. e4 e5. So until recently I haven't even taken my own advice in regard to sticking to one opening! In my defense it was usually a year or two between changes, but still...
Playing Black is hard because with White you know what your first move is going to be--you are controlling the direction of the game, at least for one move. As Black you have to consider what to play against e4, d4, c4, Nf3 and even sometimes f4, g3, b3...and it's much harder to take my recommended approach, which is designed to get in 6-10 quick moves that you know are good, saving time and energy for the middlegame. If the opponent plays 1. b3 and we have to make it all up as we go it's not very efficient, it takes time and remember, he knew what was coming and is probably prepared. So if you are not already set as Black with defenses you're comfortable with I recommend you...
SIMPLIFY YOUR LIFE
There are a number of fine repertoire books for Black out there, for example John Emms' Play the Open Games as Black, the only problem being that in 224 pages Emms only manages to get you prepared for the Open Games except the Ruy Lopez, which you're going to see plenty of if you go 1.e4 e5. If 1. d4 White can avoid your Benko Gambit, Albin Countergambit, Budapest Defense etc. with the simple 2. Nf3; or he can just go 1. Nf3.
To simplify your life I can recommend a couple of approaches, the Pirc/Modern (d6, g6, Nf6, Bg7 in various orders) against 1. e4 and everything else if your willing to play the King's Indian against 1. d4, 2. c4 for White. The other way to make life simple and reach a playable middlegame is d5, e6 and probably Nf6 as your first three moves against 1. d4, c4, Nf3 or g3. This system guarantees a fair share of the center and an opportunity for quick kingside castling, both pluses for those of us below Expert level. The Pirc/Modern/King's Indian approach has worked pretty well for me over the years, though as I said above I haven't always played it against 1. e4. But I've played the KID for most of my career with pleasure and can recommend it, and say that a lot of the knowledge transfers to Pirc/Modern positions.
I've got to run right now but I wanted to get this post up for your consideration; I haven't had time to really go into opening books that I can personally recommend but I think I'll get to it soon. I promise not to call it Part V, though--this is getting ridiculous...