This is likely an expensive argument to make and one that Tumblr would be glad to spend legal dollars to fight. Tactically, it would make more sense to be as c

This is likely an expensive argument to make and one that Tumblr would be glad to spend legal dollars to fight. Tactically, it would make more sense to be as contrite as you possibly can, play the naïve engineer who loves Tumblr to death, and ask them for permission to use the name in the domain in exchange for including a prominent banner + disclaimer at the top of the page or wherever Tumblr prefers.