Blue Kryptonite is a varietal of Kryptonite. It has no discernable effect on Kryptonians, but is deadly to Bizarros. It is not deadly to Belinda Zee, however; it seems to give her the power to create more Bizarros.
In Smallville, it removes Kryptonian powers, in addition to killing Bizarros. (As everything effects Bizarros in the opposite manner, Blue Kryptonite increases Bizarro powers exponentially—so much that their bodies cannot contain it, destroying the bizarro).