godzilla, I had that same problem, to a lesser extent, with the fold corners cracking and turning white. Since my print was already close enough to black in most places, I just used a sharpie to VERY carefully run along these edges and make them black again. You can't notice it in person.

In general, to avoid that cracking when folding thick paper just use a ruler and a dull edge (like the back of a scissor blade) to run along the reverse side of the crease over and over and over before you try folding it. Then fold against a straight edge.

Another trick I used: I added in these trapezoidal tabs shown here:

If you add those tabs, filled in with black, then fold them inward into the box, your open end will have nice rounded front edges for the blu ray cases to slip into smoothly.