Since then, I've also heard it referred to as "a bolt from the Johannine blue." Interestingly, there are all kinds of variations. Here are a few, courtesy of Google and Google Print:
a thunderbolt from the Johannine heaven
a meteorite from the Johannine heaven
an aerolite from the Johannine heaven
an aerolite from the Johannean heaven
a boulder from the Johannine moraine
the bolt from the Johannine blue
an erratic block of Johannine rock
a solid block of Johannine rock
a bolt from the Johannine heaven
a bolt from the Johannine sky
the synoptic thunderbolt from the Johannine sky
a meteor from the Johannine sky
the Johannine aerolite
The phrase is most often attributed to Karl von Hase, a 19th century German historian of Christianity in the form "an aerolite from the Johannine heaven." But I don't know what the original German was. I imagine the "bolt ... blue" variant is an accommodation to English idiom; but I don't know where the "block ... rock" variant comes from. Does anybody else?
UPDATE (7/24): Reader Kevin Snapp emailed me the original quotation from von Hase:
Die einzige synoptische Stelle, darnach `Niemand den Sohn erkennt ausser der Vater, und Niemand den Vater erkennt ausser der Sohn und wem irgend der Sohn es offenbaren wolle,' macht den Eindruck wie ein Aerolith aus dem johanneischen [sic] Himmel gefallen, allenfalls auch aus dem Gesichtskreise des Paulus.
The reference is Karl von Hase, Geschichte Jesu, nach akademischen Vorlesungen (2te Aufl. Leipzig 1891), 527. Many thanks, Kevin; also thanks to Pilgrim in the comments for attempting a back-translation.
It's interesting that some of the versions of the phrase preserve the rare word aerolith or aerolite, although the more recent versions abandon it for words more readily understandable (like meteor) or idiomatic (bolt), even if the image is slightly changed. And some scholars have obviously recast the image in terms they like better or that are more euphonious ("rock ... block"). There's a lesson here for biblical criticism, not least in the fact that the underlying meaning remains unchanged throughout all the permutations.