"And if they had an actual source, in say 1897, why not say '1897' rather than '1890s'?"

Lighter cleared this mystery up in another thread.

"OED online:

1895   Kansas City Times 10 Dec. 4/4   At the beginning of their career..they found difficulty in keeping their instruments out of ear-splitting mischief. In the language of the 'profesor' they struck many a 'blue' note."