The blockchain is like a ledger of transactions.
The block is a global decentralized spreadsheet that is stored and maintained by thousands of people (miners) across the globe who have special computers.
The blockchain keeps track of who sent how many coins to whom, and what the balance of every account is.
The blocks in the blockchain are made up of all the individual transactions sent from MyEtherWallet, MetaMask, Exodus, Mist, Geth, Parity, and everywhere else.
When you see your balance on MyEtherWallet.com or view your transactions on etherscan.io, you are seeing data on the blockchain, not in their systems.