Something else to keep in mind is the fact that all of two people are selected for Blue 21 with each board. That's four a year. It's a long shot, even if you're qualified. On top of that, it got canceled once and then reinstated for FY10. There's no guarantee that it'll even be around after this year.
Regardless of what direction you take, the selection process for the CG is extremely competitive. DCA, OCS, CSPI, Blue 21... they're all pretty tough to get into. Like the guys said, DCA is certainly possible, but I wouldn't put all your eggs in that basket.