This is a theory I have regarding how Blackbeard steals devil fruits using his Yami Yami no Mi, and why he cannot turn himself into darkness despite being a Logia.
First, I’m looking at how Crocodile’s devil fruit has a natural weakness. If Crocodile is wet, he cannot turn himself into sand.
We also know that Teach (for some reason) cannot turn himself into darkness, despite being a Logia. He is the only Logia to not have this ability, so it’s a bit weird and unexplained. I think that it’s possible that his devil fruit’s natural weakness is light. This would mean that if he is in contact with light, he cannot turn into darkness, just like Crocodile with water.
This would mean that Teach would have to be completely hidden from light, in order to fully utilise his devil fruit. This links well to how he stole Whitebeard’s devil fruit. He used that giant cloth to fully cover himself from light, and when he emerged, he had stolen the Gura Gura no Mi.
I think it would make sense that his devil fruit is able to completely absorb devil fruit abilities (hence, steal a devil fruit power), but only if Teach turned his body into darkness. This would mean that to steal a devil fruit, Teach would need to fully cover himself.
A hole in this theory is that we don’t know that Teach even uses his devil fruit to steal devil fruit abilities, but I think it’s fairly likely that he does based on its other abilities (nullifying devil fruits, absorbing etc.).
*Theory by thoepd