A method for authorization of a blockchain transaction includes: storing account profiles, each profile including an account identifier, fiat amount, and blockchain amount; receiving a transaction message, the transaction message being formatted based on transaction message standards and including a first data element that includes a specific account identifier and a second data element reserved for private use that includes a network identifier and transaction amount; identifying a specific account profile that includes the specific account identifier; identifying a risk value based on the transaction amount and at least one of: the fiat amount and blockchain amount; determining authorization of a transaction based on the identified risk value; modifying the transaction message based on the authorization determination; and transmitting the modified transaction message.