From the project's "about" page:

"This collection has been made available for use in research, teaching, and private study only. Copyrights that may exist in these materials have not been transferred to the Internet Archive. The Internet Archive does not advise as to the copyright status of items in our collections. Our terms of use require that users make use of the Internet Archive's collections at their own risk and ensure that such use is non-infringing and in accordance with all applicable laws. It is the user’s responsibility to determine whether permission may be required for a given use of these materials, or whether such use is authorized by law."

https://archive.org/details/georgeblood&tab=about

Copyright on music is one of the thornier areas of the law: to start with there's the dual nature of music copyrights (both the composition and the specific recording have a copyright). Possibly even more relevant here, sound recordings only came into the federal regime relatively recently in the 70s, leaving the works created before then to a hodgepodge of state laws.

The Archive is taking a fair use stance here, but as with some of their other projects there's a significant "ask for forgiveness, not permission" element at work as well.