White corner showing on black box with Border-radius

Since overflow: hidden; along with border-radius seems to cause some rendering inconsistencies in some engines (take a look at this), one should use border-radius on both the parent and the child elements to achieve rounded corners.

As you have noticed, you still get some wierd results with extra pixels "shining" through. Just reduce the border-radius of the child (or the other way round) to compensate this.

blockUI .overlay h1 {
    border-radius: 2px 2px 0 0;
}

I had same problem. But I solved.

.blockUI .overlay {background:#000;}

and remake some!