A big part of using things like 960 Grid and Blueprint is to build on the work of others. I'd say use the existing theme, read the documentation, and (IIRC) build a sub-theme. The documentation should, in theory, tell you what you need to know about what's in the theme.
Starting from scratch is a great way to learn theming. I'm not sure it's necessarily the best way to work every time. My starter theme is a really outdated version of the Basic theme. I don't update because I've studied it carefully, modified it extensively, and know what's in there. I tried updating once and found a bunch of changes I didn't really like. I'm also not likely to go off and grab another theme as a starting point very often, because I'd rather go from a known starting point each time.
I did recently grab an existing WordPress theme and convert it when moving a WordPress blog to Drupal. I regret it, because it has some bugs, and since many of the classes have slightly unfamiliar names, I keep getting mixed up when I try to fix it.
Bob's idea behind the Changeme theme was to create a very simple theme that's heavily documented, so that people can use it as a starting point. I've never really looked at it, but I wouldn't consider it a complete reversal.
Micah
I am developing a new Drupal theme for my site that will use the Blueprint framework.
Should I start from scratch, or should I use the Drupal Blueprint Theme?
I only ask because Bob has said in one of his videos that you shouldn't use other people's themes because you don't know what's in them ... But I wasn't sure if that was the case with the Drupal Blueprint Theme.
Thank you