Clearing off some space using an external media is a good idea indeed, especially if you have tons of huge files that you don't actually use but which you'd like to have safe in case you'd want to work on them in the future. As you noticed, DVD's are quite small...
I wouldn't recommend using an external hard disk as a more or less permanently attached work drive. That said, the black series would be overkill. It just consumes more energy and costs more per gb which are unnecessary features for a means of archiving. Instead I'd recommend getting two cheaper ones, even the green series will do, saving your projects and final productions duplicated. The idea is to always have the data in two places in case something bad happens. Normally they would be the computer and an external media, but since you want to get free space on your installed disks, two externals is the safe way to go.