I have a lot of tumblr.com feeds which are not loading at all. I’ve verified the actual tumblrs are still active, but newsblur is not picking them up at all. I tested out a couple of other web-based RSS readers, and they are picking them up and updating just fine.
Tumblr changed which user-agents are allowed to bypass their “safe mode” redirect. Wget still works, for example, but it looks like any random user-agent string Tumblr doesn’t recognize gets redirected to the safe mode warning html page.
(Near as I can tell, no major website respects the concept of “I’m asking for RSS. I’m a robot. Please don’t redirect me to an interactive javascript html page.”)
I don’t think there’s anything we can do on our side. Try reaching out to Tumblr and asking if they’ll support NewsBlur. I know that thousands of Tumblr feeds are read on NewsBlur and they have that data, so it’s in their interest to get NewsBlur fixed.
One user-side workaround is to take advantage of Tumblr’s whitelisting of Feedburner. As detailed here, adding the NSFW RSS feed to Feedburner, then adding Feedburner’s feed to Newsblur gets around the redirect problem.