I think the notion that guest blogging is anything other than a white hat is nonsense. Guest blogging is an excellent way to add value to the web.
There are some folks that that suggest that any type of website promotion is black hat, and that is just extreme hyperbole and FUD in my opinion. Often people say that because they are looking to rationalize and justify their mass web spamming activity.
My advice: Don’t spam, instead add value to the web!