At the bottom of the Safety checker, it says "Don't panic. Users sometimes post bad content on websites that are normally safe. Safe Browsing will update the safety status once the webmaster has cleaned up the bad content." As WP points out, some people might have used Google's services to host malicious content, and the checker just can't single it out.

The Safe Browsing checker recognizes a lot of other websites that post user content like Tumblr as partially dangerous, as well. This all just demonstrates that the tool isn't perfect: It's best to exercise caution even if it says that the URL you're visiting is safe.