Your iCloud Password is the same as your Apple Account password. If this is the same Apple Account used to sign in here, then that password will be the one you want to use. Not sure what you are referring to with blockchain data as that would suggest you would need a completely different Account/Password used with the app that saved the data in iCloud to access that data and it is not the iCloud password you are looking for. In that case you would need to contact the developer of the app for instructions on recovering your password to decrypt the data saved in iCloud.