This is a guide for twitter, but works for tumblr and other sites.
1) Find the word you want. For example: Clraik
So the URL you want is: Clraik.tumblr.com
2) Find a country (Europe, UK, etc) which allows for trademark of a word, in this case "Clraik"
3) Go through the very small legal mumbo jumbo and acquire proof of ownership.
4) Contact email@example.com
and file a complaint about the word you own is currently inactive, and ask if there is anything they can do.
5) You will be asked for proof of ownership as well as be forwarded forms to prove that you aren't filing a false complaint.
6) Enjoy your 20-150$ URL. (Estimate) I did this last year for a very "popular word" at the time with no issues.
This is basic knowledge, and can work for sites such as YouTube etc just don't go overboard!
----This is a non-blackhat method of getting urls.