what are the best openings for the black pieces for beginners?

Naroditsky's recommendation as a simple opening for beginners is the Accelerated Dragon. There are only 1-2 major lines and then some anti-sicilians to learn.
A lot of the time people will try to play the Yugoslav Attack and then you play d5.
The position opens up in black's favour here as black is fully developed and castled. Always look out to play d5 whenever it's good.
The main line continues like this.
There is also the Maroczy Bind. This is probably white's best response but not many people will play it at lower levels.
As you can see, you're generally aiming for the same basic setup and controlling the dark squares with a combination of the c5 pawn, g7 bishop and c6 knight. You can also use this fianchetto setup against many anti-sicilians.
This is basically the same setup without trading off the pawns.
Against the Alapin I recommend d5.
The simplest approach is to force an IQP, then you trade off pieces and win the isolated pawn and the endgame.
Job done.

I would also suggest the Sveshnikov though. This is very forcing so you will generally lead your opponent down a particular line.
Most of the time you will see this position.
The games will generally revolve around this theme of white trying to control the d5 square and black trying to attack. In this position black has the immediate threat of b5-b4, forking the knights.
Also at lower levels, a lot of people will take on c6.
This is better for black as it brings a pawn closer to the centre which will support d5.

Naroditsky's recommendation as a simple opening for beginners is the Accelerated Dragon. There are only 1-2 major lines and then some anti-sicilians to learn.
A lot of the time people will try to play the Yugoslav Attack and then you play d5.
The position opens up in black's favour here as black is fully developed and castled. Always look out to play d5 whenever it's good.
The main line continues like this.
There is also the Maroczy Bind. This is probably white's best response but not many people will play it at lower levels.
As you can see, you're generally aiming for the same basic setup and controlling the dark squares with a combination of the c5 pawn, g7 bishop and c6 knight. You can also use this fianchetto setup against many anti-sicilians.
This is basically the same setup without trading off the pawns.
Against the Alapin I recommend d5.
The simplest approach is to force an IQP, then you trade off pieces and win the isolated pawn and the endgame.
Job done.
I would recommend the Dragon over the Accelerated Dragon, it's the exact same moves, except 4... Nf6 5. Nc3 g6 rather than 4... g6 allowing the Maroczy Bind. The Maroczy Bind all but invalidates the Accelerated Dragon.
I would also recommend the Nf6 Alapin as it scores better at all levels, including 2000-2500 in Classical on the Lichess database. White actually scores as well as Black in the d5 Alapin. Unfortunately, no matter which Alapin you choose, you will need to memorize a decent amount of theory.

I would recommend the Dragon over the Accelerated Dragon, it's the exact same moves, except 4... Nf6 5. Nc3 g6 rather than 4... g6 allowing the Maroczy Bind. The Maroczy Bind all but invalidates the Accelerated Dragon.
I would also recommend the Nf6 Alapin as it scores better at all levels, including 2000-2500 in Classical on the Lichess database. White actually scores as well as Black in the d5 Alapin. Unfortunately, no matter which Alapin you choose, you will need to memorize a decent amount of theory.
I prefer the Dragon to the Accelerated Dragon, definitely.
The Accelerated Dragon is probably simpler though. Also at lower levels a Maroczy Bind will likely not be that common. A lot of players won't know about it or won't know how to manage the bind.
Nf6 is probably the best line against the Alapin. I proposed this d5 line as a simple IQP plan which is easy to understand.

Heres a video of some of the most popular openings on chess.com.
The best opening is the one you enjoy playing. So pick the one that looks fun and once you learn it it will be the best one for you. I think this is true for Black or White.

Choose openings in the order in which they gained popularity.As a beginner choose a1.e4 opening for white and decide on a 1.e4 e5 opening as black,play the Queens gambit accepted/or declined vs 1.d4,play by principle vs anything you don't recognize as black.these are your bread and butter openings.Gradually learn a single attacking line vs the half-open defenses.I think beginners should shy away from anything called modern or hypermodern,I would advise sticking with the Ruy Lopez,Italian Game,or Center Game for a long time.Until you can say why a pawn move was very good,vs moderately good,or even slightly bad,stick w/the classical openings.