The 441 chapter thread never reached any sort of consensus about the power of the Darkness Fruit ability. Now that we've had a couple of weeks to think about it, I'm hoping to continue the analysis of the Yami Yami power to hopefully gain some better understanding of it capabilities and limitations without adding to pointless versus arguments.
Origin of the Darkness Fruit:
Oda was flipping through the channels when he ran into a Nova program about black holes. He learned that black holes use gravity to suck in anything that has mass and crush it. Even the elemental particle/wave of light energy, the photon, will be attracted and compressed by the extreme gravity of a black hole. Oda says to himself, "wow that's cool! I should so make a devil fruit around that." So he comes up with the idea of a black hole black hole fruit. But that sounds stupid. So does gravity gravity fruit. So, because black holes reflect no light, he comes up with the darkness darkness fruit, which sounds quite menacing.
Capabilities of the Darkness Fruit:
1. Blackbeard's power is the manipulation of Gravity:
I'm quoting myself from the 441 chapter thread, so if you've read that already you can skip this part.
! Gravity is a force that accelerates any mass towards the central point of gravitation at a rate equal to the gravitational constant (g, which on earth is 9.8 meters/seconds² towards the core of the earth). Remember the 100x (g) training rooms from DBZ? Same idea.
! So, Blackbeards power is simply to accelerate any object and most forms of energy towards his shroud of darkness.
! For example: in his Black Hole technique, Blackbeard expands his darkness shroud through out the town and then ramps up the gravitational constant (g) of all mass in its vicinity. The buildings aren't meant to resist that kind of stress and crumble. In his Liberation technique, he reverses the (g) and all the buildings shoot up into the air.
2. As a side effect of controlling gravity, blackbeard can also drain Devil Fruit energy out of fruit users:
The explanation behind this power is that Oda figures that if a black hole can attract light photons, it should be able to do the same to devil fruit energy/particles (fruitons?). In our universe the gravitational constant necessary to absorb light would crush a human to the size of a grain of sand, but this is odaverse we are talking about.
Functionally the effect of being drained of devil fruit energy is quite different from the effect of Water or Seastone. Water cancels DF powers leaving a user weak and helpless. Blackbeard simply drains the DF power, leaving the user unable to manipulate their ability but physically quite fine.
This would have different effects on the 3 different types of DF ability:
Logia are the most effected as they lose the ability to transform or manipulate their element, leaving them as regular humans.
Zoan fruit users are an unknown. It could be like water in which Zoans transform into their most "comfortable" form, like choppers chibi form. However, there is also the possibility that the removal of devil fruit energy would simply stop a Zoan's ability to transform, leaving them in whatever form they were when grabbed.
Paramecia would lose the ability to manipulate their power. But those paramecia whose body has been transformed by the devil fruit would not revert to their vanilla state. Luffy would remain rubber (like in arlong park when he was stuck underwater) but he would lose the ability to stretch his limbs at will. This means no pistol,bazooka, or whip attacks etc, but if he has already transformed into a gear form then there is no reason to think he would lose that gear. Gears rely on Luffy being made of rubber and not necessarily on his manipulation of the rubber.
Potential abilities:
There are a number of logical extensions to blackbeards abilities that he did not make use of in his fight against ace. My personal impression is that BB went easy on Ace because he wanted him as part of his crew.
For example, the Black Hole attack could easily be a one hit kill for any enemy except luffy. Spread the shroud around and rank up the gravitational constant and any enemy could be compressed into a pancake. Luffy would be relatively immune to this because rubber resists compression (think of why large doors have rubber doorstops or how we use rubber tires to cushion landing airplanes) The only reason that the Black Hole attack is not all powerful is that it is slow. The towns people were fully capable of out running the black fog and blackbeard seemed rooted in place and immobile for the duration of the Black Hole and subsequent Liberation.
The Whirlpool attack could be used to accelerate an enemy towards blackbeards fist, causing the force of a regular punch to be multiplied exponentially.
Logically, touching the "black fog" darkness shroud should have the same devil fruit draining effect as being touched by blackbeards body. However, blackbeard only explained that technique in reference to body contact so it is possible that Oda nerfed it so he wouldn't be too powerful.
When the darkness shroud is expanded is can also be used to divert projectile attacks. Real life marksmen have to correct for gravity over long shots, so blackbeard could use increased gravity to force shots into the earth or use "liberation" to launch them spaceward. This is likely how Blackbeard defended against Ace's "flame emperor" attack at the end of the bout while his shroud was spiking up into the air.
Limitations of the Yami Yami
One unexplained drawback of the darkness fruit is why blackbeard can not cover his whole body with his black hole element. During his fight with ace, his darkness shroud only covers his jacket, bandanna, hands and feet. It's possible that if he covered his front in the shroud he would suffocate, but it has not been adequately explained.
Blackbeard can only attract and compress things with the shroud. In ch 441 you can see blackbeard using his darkness covered hands to draw away the fire from his chest after ace's attacks. After the firefly attack, he pats his burning chest down and the flames disappear. When he got hit with the fire spears, he grabbed them and they went away. After the flame cross attack you see him holding his hand in front of him and grinning. He can use his hands to compress the fire into nothingness, but his front is completely exposed to attack.
Also he is more vulnerable to standard attacks than most. By accelerating objects towards him, punches, bullets, blades etc. should hit him harder than they would others. He has plenty of defense to these attacks, though, with his shroud diverting and threatening any would be attackers.
Summary:
The yami yami is a very powerful devil fruit, but only as good as the tactics used with it. It does trump the invulnerability that logia fruits displayed thus far. However, it is also much more vulnerable to physical attacks than your standard logia.
Luffy should have some advantages against blackbeard as he is a physical fighter who will remain invulnerable to blunt attacks or compression even while nerfed by blackbeards touch.
I hope this made sense to some of you and that if you find any part of this poorly reasoned please explain why and offer your own theories.
Hell, I'm happy that you read this far in this monstrosity of a post. Be happy.