It looks like BlindDown is a bit jumpy on definition lists but not with divs (using the same text). And, yes, I've set all the margins and padding to zero ;). To be sure, this only occurs in Firefox and not in IE. (I'm using Firefox 2.x.)
I've created a testcase here:
http://www.handcoding.com/documents/testing/blinddown-test/blinddown-test.html
The steps to reproduce are listed on the page, but the gist of it is that I've assigned 'click' event handlers to elements that trigger BlindDown on either a div or a dd element (as part of a definition list). After clicking on those, respectively, the div slides down smoothly but the dd element appears to come in suddenly at the end.