The way I figure it, its own url is the best way to go if you're going to invest some time to grow it organically. This is especially important I think if you're going to be covering a unique topic.
Now, if you're going to cover mostly one topic about a site you already own it may be best to /blog/ it.
Lets imagine in Torn's case he's going to have kat start her own blog, putting it at the Kat's url under /blog/ if its JUST about kat doing stuff would be pretty smart.
If its going to cover a wider variety or stuff a new url would be my best suggestion.
