Blue Kryptonite (aka Bizarro Kryptonite)
This form of Kryptonite, also known as "Bizarro Kryptonite" because it emits radiations to which only Bizarro creatures are vulnerable, was first brought into being by Superman in October 1960 as a means of thwarting an invasion of Earth by Bizarros from the planet Htrae. By focusing Lex Luthor's "duplicator ray"--an ingenious device designed to create imperfect molecular duplicates of either animate of inanimate objects--on a mound of Green Kryptonite, Superman produces a new, imperfect form of Kryptonite that is harmless to Kryptonian survivors but deadly to Bizarros. One unfortunate Bizarro creature, a Bizarro-Supergirl, succumbs to the fatal effects of Blue Kryptonite poisoning in October 1960 (S No. 140: pts. I-III—"The Son of Bizarro!"; "The 'Orphan' Bizarro!"; "The Bizarro Supergirl!"). (TGSB)
Unlike other forms of Kryptonite, Blue Kryptonite radiation is not blocked by ordinary lead. However, an imperfect duplicate of lead created with the bizarro ray will block Blue Kryptonite radiation as is seen in November 1961 when a race of Blue Kryptonite creatures who live inside the Bizarro world of Htrae invade the surface of the planet (Adv No. 291/2: "The Bizarro Perfect Crimes").
One apparently apocryphal account has Bizarro #1 killing himself with a small piece of Blue Kryptonite so that he might be a perfect imperfect duplicate of Superman. This is necessary, as Bizarro reasons, since Superman is alive, he should be dead (S No. 423: Sep 1986, "What Ever Happened to the Man of Tomorrow?").