Mounts are traditionally affixed by slightly moistening them. But if you use too much water, the paper will buckle. And if you don't apply enough water, the mounts wont stick properly. In addition, the mounts will not always stick to coated papers.
Now we have the new DAVO easy, a self-adhesive mount. Hence, no need for water anymore, no buckling paper and it always sticks! Black stands for black background.