What exactly is a "blockchain developer"?
In bitcoin core &quot;block chain&quot; is used at lot of places and all the pull requests to change it since 2014 got NACKs so never changed
https://github.com/bitcoin/bitcoin/pull/5242
Satoshi mentioned 'timechain' : https://bitcointalk.org/index.php?topic=382374.0
The timechain is a tree shaped structure starting with the genesis block at the root, with each block potentially having multiple candidates to be the next block.  pprev and pnext link a path through the main/longest chain.  A blockindex may have multiple pprev pointing back to it, but pnext will only point forward to the longest branch, or will be null if the block is not part of the longest chain

Isn't &quot;blockchain&quot; just a simple word to refer to the &quot;chain of blocks&quot;

Yes
According to a definition mentioned in one of the slides used by Andreas in video: https://youtu.be/FYo5E7zT-vM
Blockchain is the public ledger where the network records are written.

Why is there such a focus on something they refer to as &quot;blockchain&quot;, sometimes &quot;Blockchain&quot;, as if this is some sort of &quot;service&quot; that magically exists somewhere on the Internet

Not sure when people started using the word &quot;blockchain&quot; as technology, became a buzz word and now used for lot of things related to applied cryptography and cryptocurrencies.
There are few more concepts which use 'chain' in it: sidechain, statechain, spacechain, softchain, etc.
So I am assuming few developers that are involved in something related to cryptocurrencies categorize themselves as Blockchain developers.