Whats a good overall opening for black??

I've always had trouble playing black (Never had an issue when I play white) when it comes to the opening because I always have to react to what my opponent chooses to play as their first move. Right now, when my opponent plays e4 or d4 I play c6, the reason I do this is to try to enter the Caro-Kann or the Slav Defense. However its quite hard for me to learn both these openings at the same time and also I don't know if my opponent will even choose to play into the opening I want. Whats a good overall opening that I can play for most if not all first moves? The last time I checked my stats I had a 49% win rate with black because I tried experimenting with some other openings like the Dutch Defense and the Sicilian Defense. Please help me!!!

A 49% winrate is good with Black. It is impossible to maintain > 50% with Black unless you have a bad winrate with White, because if your overall winrate is > 50%, you will gain rating and play stronger players until it isn't anymore. Unless your name is Magnus Carlsen or Hikaru Nakamura.

There are no opening shortcuts for Black and trying to use one will just create worse problems than not knowing what to do in the opening. At 600 rapid, openings are basically irrelevant. If you quickly feel lost, that is normal. Chess is hard.

Let me preface this by saying... you're 600 elo, you don't really need to go in depth in the opening, you need to work on tactics. Just learn the first 4-6 moves and play the positions from there. You can learn that in about 30 minutes from any introductory youtube video on the opening. But regardless I'll answer your question in a serious way, and I'm assuming you're able to interpret what I say and apply it to yourself where it makes sense without getting lost and crashing into a mountainside in terms of your chess trajectory. 
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So what you should really do to solve this problem is just focus on your e4 defense as black and try to mostly ignore d4 until you're ready for it. Because e4 is way more common statistically than d4, especially at your level. And for any other first moves, like c4 or Nf3 or g3 or Nc3 or whatever else... you should completely ignore these, devote absolutely no time to them.

If you want to play good defenses they'll need to be targeted specifically at your opponents moves. Which means for a while you have to tolerate not knowing exactly how to play against the less common moves.

- For now just learn the first 6 or so moves of your d4 opening... and then focus exclusively on e4. You don't have to know the theory deeply to play an opening. If you just know the first 6 moves... and that's not hard to learn, you can reach a mostly playable position and wing it from there. This isn't ideal but it's the best you can do until you have time to buckle down and study the lines in more depth.

- It's always best when learning theory to start with the most common lines. As black this will be e4. You should master your e4 defense before worrying too much about the others. And within e4 focus first on countering the most common main lines.

If you're curious about move percentages at your elo refer to the lichess analysis board.

This isn't actually true because as you rise in elo the frequency distribution of players is weighted toward the lower end of the matchmaking systems elo search range for you... so you get matched with lower rated players more often than with higher rated players.

That is true but the effect is pretty subtle unless you are way way up there.