With blockchain, there is no need for a centralized source. Computers are connected to a network that provides all accounting for everyone on the chain. If one computer fails, the others serve as backups. All computers connected to the network aid in the development of a communal public-facing ledger. If one fails, the others pick up the slack. Instead of double-entry bookkeeping, blockchain is just X-entry bookkeeping, where X equals the numbers of nodes on the chain.