Snapshot option: There is the option of giving every address who holds BLK on a certain block/or time the same amount or proportional amount in Blacknet. Then the people who want Ercoin can burn their BLK for Ercoins.
Burn opt-in option: There is also the option of letting people opt in to Blacknet by burning the amount of Blackcoins that they want to participate in Blacknet. But this comes at a cost to the person participating in not having the Blackcoin that were burned anymore. Ercoin can also give Ercoin to the people in this list who already burned BLK for Blacknet, or Ercoin can ask people to burn BLK again for Ercoin's own list.
No-burn opt-in option: There is also another option of letting people opt in to Blacknet by signing that they want to opt in to blacknet with the address that contains the Blackcoin balance that they want to participate in Blacknet without having to burn it. They would have to submit the signature and what they are signing somewhere, for example in a website or something that Rat4 makes that can allow this. Then at a certain block or time it checks the balance of the addresses submitted.
Both the opt-in options come at an additional cost of not giving people who didn't opt in any coins. Perhaps they are exchanges, so people on exchanges wouldn't get any coins. Or they are on vacation and didn't follow the information or anything and would not get any coins.
If he decides to opt-in to get Blacknet? This will result in a lower coin amount than blackcoin currently has.
The option I would prefer is to just send BTC to Rat4 and get some Blacknet but I don't think that will happen.