Why does Blockchain (the company) open source their code but not license it?
If they haven't put a license, it means it is "All rights reserved" You can refer to these questions about it:

https://stackoverflow.com/questions/4007674/whats-the-default-license-of-code-published-at-github

https://softwareengineering.stackexchange.com/questions/148146/open-source-code-with-no-license-can-i-fork-it

https://opensource.stackexchange.com/questions/1720/what-can-i-assume-if-a-publicly-published-project-has-no-license

It probably means "You can use it for yourself, and for small projects. However, if we want, we may shut down all your projects containing our code."