my issue with this is that it would make no sense to reclassify the darknessfruit as a logia, if it wasn't one. it doesn't behave like a logia, blackbeard cannot turn into the element. it looks like a paramecia. if you wanted to hide a secret godfruit, have people not go after it, you wouldn't list it as a logia. also i think blackbeard will eat orochi's devilfruit. i see him with the 8 heads already...
we will get a second hidden mythical zoan though: the baku baku no mi is in actuality the bakufruit (the dream eating yokai). wapol has a big role to play still.
Here's why it would be a logia:
The theme of the weakest vs the strongest
Nika fruit, gomu gomu, a paramecia
Imu fruit, yami yami, a logia
Paramecia is the weakest, logia is the strongest
Paramecia and logia are put into a neutral field, zoan, and fight it out
Weakest overcomes the strongest