Blockchain is a technology with which decentralization can be possible. decentralized applications can be built. Cryptocurrencies can be created. Transaction can be made peer to peer in a trustless way where you need not have to depend upon a third party. Apart from the financial use case it has many other use-cases like authentication, third party verification, auditing, identity verification, supply-chain, logistics and many other variety businesses can be implemented with blockchain technology. So this is basically the foundation upon which decentralized application can be built which can work in a trustless way.
Blocktrades is merely an exchange which facilitates the exchange to cryptocurrencies. However it has very limited number of cryptocurrencies and it is popular only within steem community as it has steem & sbd and most people convert their steem and sbd to other crypto assets.