If you are recommended a blog you don’t agree with, for example, if you’re anti-sjw and you’re recommended a feminist blog, the reason is quite simple.
Tumblr creates your recommended blog based on blogs or tags you interact with. If you interact with a feminist, or multiple feminists, Tumblr’s algorithm assumes you like that topic (through tags and keywords) and recommends you similar blogs.
I don’t know the exact algorithm or what it looks at, but I can combine what I know about recommendations and algorithms with my experience.
TL;DR: If you interact with a tag, blogs related to that tag will be recommended to you. If you interact with a blog, that blog might be recommended to you.
ifesbob 