I've never seen "The Life And Death Of Colonel Blimp," but it hasn't exactly gone unnoticed. Roger Ebert chose it as one of his "great movies" (along with a couple hundred others). It was directed by the British director Michael Powell, whose career was effectively killed when he came out with "Peeping Tom" -- which some critics also consider a greatly underappreciated film. (I was somewhat disappointed by "Peeping Tom," but my expectations were very high when I saw it.)
One of my favorite overlooked/underappreciated films is "The Late Show," a 1977 Robert Benton film starring Art Carney and Lily Tomlin. It's an offbeat variation on film noir, and reminds me of Robert Altman's "The Long Goodbye" (another favorite of mine).
Although certainly not overlooked, "Saturday Night Fever" is, I think, underappreciated by those who just saw it as a vehicle for John Travolta's dancing. I think it's an excellent examination of the desperate desire to feel significant in modern society. (Other fans of the film, including Gene Siskel, have had their own reasons for loving the movie.)