This suit seems to be quite enigmatic in its effect on chocobo digging, so if anyone still does it please post any findings you get while using it.
Also be sure to include your digging rank and the stats on your bird if you are digging with a raised bird.
I am A25 with a Burrow+Treasure Hunter bird, without the body I have about a 82% dig accuracy.
Once I put the body on, like others have noticed, my dig accuracy plummets to about 33%.
It also seems very streaky, like digging used to be once upon a time. I'll go 7+ digs with no result, then get like 3-4 items in a row.
Furthermore my bird's Burrow ability seems to not proc often at all, if ever, when I use the suit. Very odd.
Outside of that I haven't noticed anything different in terms of drops yet except getting a Heavy Metal in West Ronfaure, which as odd.
The Black Chocobo suit is obtained from the New Years event for those who don't know, specifically the "Trade any random items to the procession of NPCs wandering around outside" one. Took me about half a stack of shihei to get my body.
Right now my suspicions are the suit does one of two things:
One: Simulates digging with the Treasure Hunter ability birds can get, allowing players to have access to the ability without raising a bird. If you are digging with a raised bird that already has it, the suit might do nothing.
Two: It acts like the fisherman's apron, and will often 'cancel out' low quality digs in order to lengthen your stamina and bias your digs towards better loot.