For anyone in the same situation it seems you can't customise the CaptionBlockRegion itself. It's style is taken from the captions themselves and applied the CaptionBlock. You can change the LayoutTemplate of the CaptionsPresenter which of course
affects the size the CaptionBlockRegion, but padding, background color etc. still come from the captions themselves.
Through the captions you should be able to set the following (I've not experimented with these, just grabed them from the SMF source code):
I hope this helps someone.