What version of CSS3Pie are you using?
I don't know how blockUI works, but it is possible that it hides its pop-ups from the top of the screen or something like that until they are used.
If so, and if you are using the first version of CSS3Pie (beta 1), there was an error in this version that triggered a Javascript error when trying to access an element with a negative position. This bug has been fixed in beta 2.
If you are using beta 1, you must return to the CSS3Pie site and install beta 2.
Finally, you asked on the CSS3Pie website - the forums are quite active, and the developer himself answers many questions.
If it still doesn't work, I find it pretty close to releasing the third beta pretty soon.
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