Backup the whole dir, from the github:
Byteball uses a single extended private key for all wallets, BIP44 is used for wallet address derivation. There is a BIP39 mnemonic for backing up the wallet key, but it is not enough. Private payments and co-signers of multisig wallets are stored only in the app's data directory, which you have to back up manually.
Should I wait until the transefer arrives or I can backup now and the transfer will be backed up anyway?
Make a backup now and once again, after the Bytes and Blackbytes were distributed!
The Blackbytes data is only stored locally, so you need to backup that local data every time you have received any!