I have limited knowledge of tumblr (i.e. I don't have an account there), but if he setup his tumblr page to redirect to an external site that uses the tumblr API (which it appears to do, see: http://www.tumblr.com/docs/en/custom_domains
), there is nothing suspicious about that.
The fact that...ahem..."questionable" material is shown there is irrelevant, although said material seems to be common on tumblr from what I've seen.
That said, you should always be suspicious of sites you've never visited before. If you click around enough, you WILL find malware being pushed somewhere.
Edited by keyboardNinja, 15 August 2011 - 07:22 PM.