Is Blockchain an open source technology or is it owned by a company?
I didn't see it spelled out but Blockchain.com website appears to belong to a company.
There also seems to be different implementations of blockchain databases/ledgers.
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Blockchain is some concept. There are different implementations of blockchain. Most of them open source. The most notable one is the cryptocurrency Bitcoin with their reference implementation -Bitcoin Core.
Blockchain.com is just the company with the website which provides some services related to the bitcoin cryptocurrency and owns very attractive web address.