I've been using a really great library called
TumblrSharp for some time now by the fantastic
Turboronin. However, it's been dead ever since 2014. I've since found a few bugs that needed fixing, as well as lacking documentation.
I've "forked" the project into my own GitHub repo
that you can check out here. I'm going to keep maintaining it since it's the only well-functioning and fully featured C# client out there, and I love the language.
I've also written some
cursory documentation to get started, since there isn't much useful info on the original site, and a lot of figuring out how the API worked was a combination of capturing the traffic from the Tumblr app, and reading the source of the C# library.
Feel free to contribute and give me suggestions and ideas :)