A link has long been posited between the worlds of Judaism and of the band the Grateful Dead. Now it’s the topic of a weekend event commencing tonight at the Isabella Freedman Jewish Retreat Center in Falls Village, Conn. Playfully called “Blues for Challah’’ (a pun on the 1975 Dead album “Blues for Allah’’), the confab will feature Grateful Dead-inspired meditation and prayer, an examination of Jewish themes in Dead lyrics, and guest rabbis who will muse on their experiences straddling both worlds.