You're right, margin-top only works for left or right floated titles but not for centered (non-floated). Use this instead:
But if you're using a border around the overlayed blog title (which you are not, but others might), then the border would stay on top. To avoid that, the top padding would have to be applied to the parent container instead, via HTML/CSS Inserts -> CSS Inserts
height: XXXpx; }
with XXX being the [desired image height] - [padding-top value], so if image height is 150 px and padding-top 30px, this would be "height: 120px;"
I am updating the "centered" example in the Theme Options...