@DJRob have you asked Fitbit? I'm not sure how many of the awesome Fitbit users will be able to explain this to you.
I'll take a stab.. Fitbit decided first to release it as a package and only produced a certain number at first to test the market. I did say I was stabbing at an answer.
I did do a search of the feature request forum and found somebody else wondering the same thing, but not sure if they are asking for this option.
I suggest leaving your vote over here..
https://community.fitbit.com/t5/Feature-Suggestions/Blaze-gunmetal-frame-and-band-available-separate...BTW I moved your post to the Blaze out of the community discussion.