I had some difficulties using Union in my knife blade model so I did some experiments with a simple minded model of two blocks.
First I translated, the lower left corner of the green block to the closest corner of the blue block. Then did a Union to make them one shape. Curious that the new Shape is of mixed colors with only one face changing to blue? I would expect a single color or that prior colors are maintained.
Not a big deal! But I know that you guys are perfectionists so I am reporting…