How did Blackbeard steal Whitebeard's power?

Answer 1

The answer can be linked to Chapter 676.

Important points are:

1. After eating the giant candy, Smiley "dies" and is reborn as Shinokuni. It loses its Devil Fruit in the process, which is reborn into a nearby apple; and

2. This marks the first time a Devil Fruit is seen regenerating after its host dies.

Here are additional points with which we can use to come to a possible answer:

1. Blackbeard had to conceal himself before he "stole" Whitebeard's power, showing that they have a better understanding of how the Devil Fruits work and would not want the world to know about it.

2. The Blackbeard pirates' doctor, Doc Q, if I'm not mistaken, carries apples with him most, if not all of the time.

3. Eating two Devil Fruits will kill a person, according to Master Oda(Well, he actually said the person would explode like a bomb).

4. The Yami Yami no Mi, which is Blackbeard's power has the ability to suppress or nullify a Devil Fruit ability as long as the user remains in contact with the victim.

5. Whitebeard is dead.

So here's my conclusion:

The Blackbeard Pirates knew before hand that once a Devil Fruit user dies his/her Devil Fruit powers don't just disappear but instead would be reborn into a nearby fruit(where they got the information, I wouldn't know for now). So during the war and upon Whitebeard's death the BB Pirates rushed to his body. They could've brought any fruit but I would like to assume that they still used Doc Q's apples in the process. So having those apples near Whitebeard's dead body, the Gura Gura no Mi was reborn thus giving Blackbeard the opportunity to eat the new Gura Gura no Mi fruit. Of course he should've just exploded on the spot but, remember, he still has the Yami Yami no Mi as his power. This enables him to suppress the other Devil Fruit power to give way for the new Devil Fruit power he just acquired.

It is also good to note that the Blackbeard pirates have began searching for other strong Devil Fruit powers to absorb during and after the timeskip(just couldn't remember what chapter that was mentioned). So most probably when he meets up with Luffy he would have numerous Devil Fruit powers(this just reminded me of Sylar from Heroes).

Answer 2

The Yami Yami no Mi is a Logia -type Devil Fruit that allows the user to create, control, and transform into darkness at will, making the user a Darkness Human and give the power to absorb everything into a black hole

the way blackbeard stole whitebeards power is he used black hole to absorb whitebeard power