It's not a very common usage, in the sense of "regain consciousness." It's literally a reference to a movie editing effect. A scene "fades out" when it gradually turns to black, and then the next scene "fades in" as the black gradually turns into a new image. It's not a stock phrase. It's something a writer might well use as one of many ways to describe regaining consciousness. It also seems to me that it would usually used to describe a drowsy, in-between state in which a person repeatedly "fades in and out" of consciousness. That is, the usual usage is "fade in and out," not just "fade in." A person who simply regains consciousness--a boxer who has been knocked out, for example--doesn't "fade in," he "comes to." Rather than invent one, I'm going to use Google to find one. Since it's relatively recent, I won't search Project Gutenberg texts which are mostly from before 1923. I'll try a Google Books search. If I include "consciousness" in the search I am overwhelmed with film references! "patients may seem to fade in and out of consciousness" "Bob Droste has watched burn patients recover consciousness. "They fade back into the picture. It's not out one day, awake the next. They sort of fade in, they know a little bit more, a little bit more, a little bit more." "At most, there are snatches of my conversations with Beckah and Virginia, but these fade in and out of my consciousness like barely remembered dreams." "Although I was quite comfortable, I began to fade in and out of consciousness as my usually law-abiding husband pushed harder on the accelerator and forced the speedometer ever higher."