I've landed in this thread because 1P doesn't generate a new login item from the Tumblr login page. I suppose it is a related issue.
Chrome version:34.0.1847.131 m
Chrome extension: 184.108.40.206
1Password main program: 220.127.116.110 (on Windows 8.1, Spanish-Spain).
I split your posts and moved it to the 1Password 3 extensions forum. The previous thread was referring to the 1Password 4 for Mac extensions, which isn't related to the Windows extensions 3.9.21.
You'd need to manually save the data on Tumblr. Here's how:
It should fill in properly the next time you visit the site.