I am going to ignore BB's strength itself and focus only on the Devil Fruit which he has eaten.
Examining the basic Logia fruit which we've seen so far:
Smoker user of Smoke can:
- Control smoke
- Create smoke
- Turn his body into smoke
Smoke is created by lots of particles which while in reality has no real physical threat except choking someone (lack of oxygen and dust in lungs), it can be compressed and 'densed' enough to block a person. Not possible in real life but makes sense.
Ace user of Fire can:
- Control fire
- Create fire
- Turn parts of his body into fire
- Can burn anyone or anything that he touches
Devastating effects of fire are known. This guy can create fire at will and thus set anything he wants on fire. He can also 'throw' fire like projectiles.
Crocodile user of Sand can:
- Control sand
- Reduce earth to sand
- Create sand (to a certain extent this would derive from the above)
- Turn his body into sand
- For some reason suck the moisture out of anything
Sand are tiny earth granules which also have devastating effects. Sandstorms occur frequently and cause huge disasters. Controlling sand gives you the capability to both create these 'natural' disasters as well as 'create' quick sand and use sand to choke people as smoker can.
Enel user of Lightning can:
- Control electricity
- Create lightning and other manifestations of electric power
- Turn his body into pure electrical energy
- Strike at incredible speed due to the properties of electricity
- Can shock anyone or anything that he touches
Electricity shocks people. A current large enough can kill, burn, destroy just about everything. The power and speed of such a fruit is obvious.
Aokiji user of Ice can:
- Control ice (?)
- Create ice
- Turn his body into ice
- Can freeze anyone or anything that he touches
A common manga/comic idea is that anything frozen can be broken at much greater ease than if it were unfrozen. I doubt freezing a titanium door will make it any easier to break but i'm sure Sub-Zero can prove otherwise. If if this is not the case, simply freezing a person induces hypothermia which can cause death.
Now we have Blackbeard a user Darkness. Basic pattern matching would make me assume that he has the following powers at the very least:
- Control darkness
- Create darkness
- Turn his body into darkness
But unlike all previous fruits, darkness itself does not exist. In itself darkness is lack of light. Of course darkness can have analogous or metaphorical meanings such as the induction of fear and of disorientation. This would be an excellent espionage, stealth or defensive mechanism much like attacking at night or during an eclipse. (more the latter since one would not be expecting an sudden eclipse). The creation and control of darkness per se is the same thing, since the only way to control darkness is to either make it exist or destroy it. I cannot see how, darkness in itself can be used as offensive weapon. The darkness power in itself must have some other meaning such as shadows or fear or manipulation of the void. Some have said that fire will be destroyed by darkness. However fire is a simple manifestation of energy which is transformed into heat and light energy. Simply transforming putting sheet over fire will not make the heat go away, and in itself the destructive power of fire is not the light but the intense heat. Extreme light will blind you at most. Fire will destroy you.
I would therefore have to assume that the darkness power is some sort of way to suck the life right out of somebody, filling their insides with nothingness/emptiness/void which would put them in a helpless, weak position where they are more open to attacks.
If it had not been done before with Blueno I would have said that the darkness would created a sort of 'black hole' which would allow for faster transportation and to reduce attacks into nothing by sending them into the darkness.
Finally it could involve shadow manipulation, but that hardly seems something worthy of spending time looking for.
Then again, this is Oda, I'm sure he'll come up with a single bright idea no one thought of which will kick everything's ass!