Maybe the Darkness Fruit is another secret Mythical Zoan?

There are theories about this since Luffy's Nika reveal that BB's fruit is the evil Nika, a Darkness God to oppose Luffy's Sun God.

Even though I wouldn't like it to be another MZ, I can imagine this being true since we already know that something is off about this Logia fruit because it can't make the user invincible.

I guess its awakening has the properties of a Logia fruit and that's why it was classified as a Logia. Luffy's powers are similar to a Paramecia awakening, likely that's why it was reclassified as one.