PS CS6 align/blend errors

I do "stacks" of macro images, take up to 30 images of a flower focusing at different parts of the flower in each, to get an image that is fully sharp.  I load them all into lightroom first, and then "edit in PS, load images as layers."

When all images are in CS6, I highlight all the layers to be included in the blend/align process.  I click "edit align" and then "edit blend", and wait while PS blends all of those layers together.  The resulting image will be a blend of all the sharpest points of each layer, thus a fully sharp macro image/

Have been doing this for 3 years in PS and never had any problems.  Suddenly when I was doing a stack of 25 images of white tulips against a black background, the resulting blended image was totally blown out white, OOF, like a huge splat of white on a black and slightly greenish background.  The images did not blend properly.

I've done many tests of stacks using color images over the past few days, meaning NOT white subjects on black backgrounds, and they work fine.  It is specifically a white object against a black background stack that is not working.  It doesn't matter whether I try to stack 3 images or 30 images, it does not work.

There is something wrong in the blend part of the process, of specifically images of white objects on black, that isn't working.