is this midnight blue cardstock white core? Does the outer layer crack when folded in half?

This is a solid core stock for which the midnight blue goes all the way through the stock.  We recommend scoring all card stock before folding to reduce or elimate cracking stock.

What type of printing can be done on this paper lazer or ink jet?

Although, this stock is both inkjet and laser compatible, due to its dark color, printing direct to this card stock will not have readable results.  There are some printers out there that have a white ink that would show up on this stock. But traditional inkjet and laser printing will result in muted and faint color of the print.  We generally recommend doing the printing on a slightly smaller size and  lighter color stock or paper and layering it on the Midnight blue card stock.