Blackcoin switched to proof of stake a long time ago. There is no pool where you can mine Blackcoin.
For helping securing the network you can hold blk in your wallet and stake your coins. As reward you shall receive blk.
There is no fixed percentage.
The reward for staking is 1.5BLK + tx fees. This is fixed. The block target is 64 seconds.
The likelihood of you being the staker in any block and earning 1.5BLK is dependent on the amount of coins you hold in your wallet (your weight) vs the total number of coins which are being staked (network weight). Think of it like a lottery where the number of tickets you hold increase the chance of you winning the prize in a competition which has to have a winner and that restarts every 64 seconds.
This varies over time so there's no definitive answer. Generally speaking though people earn between 5-8% a year if staking 24/7/365.
it will be 4%