black market win percentage

No, they won't. The BM is a casino/gamble and they won't introduces this simply because it will decrease sales.
Say you have a gun with a win chance of 0.5% (meaning that you should get the weapon in 200 crates (which is not guaranteed because you have the 0.5% chance every time you spin)). For some players, this will be why they won't buy the crates because they think the odds are too low.
I get why you asked it but they won't add it, sorry

(numbers used are not correct in any way)

try to spin when servers are emty