I think that Samus's dark outfit is meant to be the Phazon Suit from Prime 1.

[Boolossus]

Most of the time when people refer to this suit, they seem to refer to it as the dark suit from Echoes. I think it more closely resembles the Phazon Suit from Prime 1 more though. Just take a look.

Left side=Dark Suit from Echoes
Middle=Phazon suit from Prime 1
Right side= Samus's Dark costume in Brawl.

Similarities between the Phazon suit and the Dark costume:
-Shoulder pads are same shape, the shoulder pads on the dark suit are thinner.
-Visor is same shape, the visor on the dark suit is less round.
-Shoulder pads are a darker color than the rest of the suit, the shoulder pads on the dark suit are the same color as the rest of the suit.
-Chest plate is shaped the same, the chest plate on the dark suit is more flat.
-Pointed knee plates, the knee plates on the dark suit are flat.
-Armor plating is the same, the armor plating on the dark suit has a more plain style.

 

[GotACoolName]

The Brawl outfit is brown mixed with gray, has a more orange than red visor, and is covered in those red dots that the Dark Suit has in Echoes. The only way it resembles the Phazon Suit more is the shape, and the only reason it resembles the shape of the Phazon Suit is because only Wario and Pokemon have alternate costumes that aren't just palette swaps. On the Phazon Suit, the helmet is part of the gray section. In that video, you can see that the helmet is brown, and the gray sections are mostly in the legs and chest, just like in Echoes. It's definitely the Dark Suit.

 

[Phazon_Dude]

Yeah.. thats defiantly Dark Suit.. Samus has no black on her armor or arm plates, more of a grey and dark brown.

 

[Dream Chaser]

Agrees, Dark Suit

 

[X Megadragon]

It's definitely the Dark Suit. Remember this is a costume change, so they couldn't change the model of Samus or edit her geometry to make her shoulder pads resemble the Dark Suit... but the palette is exactly the same.

 

[Boolossus]

Her shoulder pads are darker than the rest of the suit in Brawl, just like her shoulder pads are darker than the rest of her suit in Prime 1.

However, in Echoes, her shoulder pads are the same color as the rest of her suit.

The colors for the dark suit don't match perfectly with the brawl costume either. And when you take into consideration that the dark outfit in brawl matches neither the dark suit nor the phazon suit in color, then all you have left to base the similarities on is the shape of the suit...

Even if the shape of the suit wasn't meant to be changed with each costume, that doesn't change the fact that the shape still resembles the phazon suit more. And if they could change the costumes for Wario and Pokemon Trainer, then why not change the costume for Samus to make her suit resemble the shape of the dark suit, and even throw in another costume change that resembles her light suit?

 

[Phazon_Dude]

....OK, see how Samus' shoulder pads, outer legs AND head all have the same dark BROWN color... Phazon suit has a silver head matching her armor and then PITCH BLACK arms and outer legs. The shape is not the Phazon suit's shape its just the original Brawl shape that Samus has for all her color changes.

In the bright environment of this stage you can easily see all the colors match exactly to the dark suit (mainly by the helmet).

http://youtube.com/watch?v=NSVdgyNPGRM

Hey, I would also have preferred for Samus to have a Phazon Suit color scheme (just check out my name) but its the dark suit.

 

[GotACoolName]

And when you take into consideration that the dark outfit in brawl matches neither the dark suit nor the phazon suit in color

Hmm, let's see...

Phazon Suit: Gray helmet
Dark Suit: Brown helmet
Brawl Suit: Brown helmet

Phazon Suit: Orangeish red visor
Dark Suit: Orange visor
Brawl suit: Orange visor

Phazon Suit: Black arms and shoulders, red spot on hand
Dark Suit: Brown and gray shoulders and arms, red spot on hand
Brawl Suit: Brown shoulders and brown and gray arms, red spot on hand (difference from Echoes suit in shoulder color homo/heterogeneosity is likely only due to the shoulders' shape differences)

Phazon Suit: Gray chest area
Dark Suit: Gray and brown chest area
Brawl Suit: Gray and brown chest area

Phazon Suit: Gray and black legs
Dark Suit: Gray and brown legs
Brawl Suit: Gray and brown legs

Phazon Suit: Dotted with black protrusions/spots
Dark Suit: Dotted with red spots
Brawl Suit: Dotted with red spots

Honestly, I don't understand how you can't notice the blatant resemblances the Dark Suit has with the dark Brawl suit, unless you have a visual deficiency or something. And why did you edit the video out of your original post? It gave a more clear picture of the suit in Brawl than any of the other screenshots in this thread.

 

[Boolossus]

Actually I do have trouble with some colors... blue purple, red green, yellow orange for example. Okay, I'll take your word for it then.

 

[Nocturnal~Dragon]

I say its the dark suit, it would have to be black for it to be the Phazon suit, the suit in brawl is more of a dark brown color like the dark suit

 

[Locke66]

Definitely the dark suit. Her midsection isn't red, and her shoulders are pretty brown looking to me. Her arm cannon stays green, too. If it was the phazon suit, it would be blue, right?

 

[Phazon_Dude]

Case closed.. pretty much!

http://www.brawlcentral.com/content.php?p=trophy/metroid/samus_dark

 

[GotACoolName]

Her midsection isn't red

That's only part of the official concept art of the suit you see in the original post. If you've played Metroid Prime and gotten the Phazon Suit, you notice that the redness is not there.

Her arm cannon stays green, too. If it was the phazon suit, it would be blue, right?

No, even with the Phazon Suit, the arm cannon normally has the lustrous green color. The concept only shows it as glowy and blue like that because,

Spoiler

during the final boss in Metroid Prime, you can only kill MP's second form through the suit's Phazon Beam, which is activated by standing in puddles of Phazon during the fight, and when you're in one of those puddles you enter "Hyper Mode" and your arm cannon morphs into that Phazon form which you see in the concept art.

 

[kin3tic-c4jun-3]

It looks like the Phazon Suit because it's the same shape and a dark color. No one's going to pay attention to the subtle color differences lol.

It looks like: Colors from Dark Suit, Shape of Phazon. It's not rightfully the Dark suit as the defining feature of that suit was the shoulders.

And in Prime 1 the helmet is not grey, it's quite blacky.