Now, speaking on blockchain, we should know that it is a technology that allows a quick, reliable, clear, concise and very secured way of exchange of digital goods and this includes money, equipment, properties and other goods.
Going further to speak on cryptocoins, they are the currencies which help in these transfers and serve as a general currency for all nations so you don't bother much in exchange. Now, Steemit is a blockchain because it also help in exchange of goods and services and its own currency is the Steem and SBD also called Steemdollars. So this is the reason for it being called that.
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From @john-snow