This is typically called a "microsite" and is not a black-hat strategy at all. It is more of a marketing tactic--to build a good, quality site with lots of content on a keyword rich domain name. Typically, many companies will purchase keyword rich domain names and not use them, they will just redirect them to their main site: like the way Books.com redirects to Barnes and Noble.
If the microsite only has thin content and doesn't have quality content, it could be considered a doorway page or doorway domain--but if it does have enough quality content on it then it would be considered a microsite. It still needs to have a good amount of quality links pointing to it, as well, in order to help the other site it points to. But, if the microsite is working, then the main brand site doesn't necessarily need to rank well. It gets its traffic from the microsite.