Basic theory rundown: the most common two scales in Western music are the major and minor scales. Songs written in major keys tend to be described as positive or light--not always, but generally speaking. Think of "Only the Good Die Young" or "Uptown Girl"--or most pop music, really.
Minor keys are often described as sad, somber, haunting, or dark. It's less common for a pop song to be wholly in a minor key; minor tonality in pop music is usually used to provide contrast to the rest of the song: "Movin' Out (Anthony's Song)" is primarily minor up until the car-revving part at the end, and the moody-sounding piano interlude in "Piano Man" is minor, while the rest of the song trends towards major.
The lyrics make reference to a "sad composition" (probably referring to this song, which is likely in a minor key, too), and a "blue situation"--the blues scale, which most blues musicians base their solos on, is a variation on the minor pentatonic scale. And in classical music, variations are when a part is repeated, but altered during the repetitions; so, a minor variation would sound familiar, but darker than its major counterpart. The lyrics indicate that, like a musical variation, this is an ongoing and recurring thing ("ain't nothing new," "more of the same thing," "it's just a part of the pattern," "it's not as though I don't know the condition").
So he's saying both that it's just a slight (minor) change (variation) in his condition, and that he's in a dark, brooding, minor-keyed mood, and a recurring one at that.
There's an underlying musical pun here, too:
Basic theory rundown: the most common two scales in Western music are the major and minor scales. Songs written in major keys tend to be described as positive or light--not always, but generally speaking. Think of "Only the Good Die Young" or "Uptown Girl"--or most pop music, really. Minor keys are often described as sad, somber, haunting, or dark. It's less common for a pop song to be wholly in a minor key; minor tonality in pop music is usually used to provide contrast to the rest of the song: "Movin' Out (Anthony's Song)" is primarily minor up until the car-revving part at the end, and the moody-sounding piano interlude in "Piano Man" is minor, while the rest of the song trends towards major.
The lyrics make reference to a "sad composition" (probably referring to this song, which is likely in a minor key, too), and a "blue situation"--the blues scale, which most blues musicians base their solos on, is a variation on the minor pentatonic scale. And in classical music, variations are when a part is repeated, but altered during the repetitions; so, a minor variation would sound familiar, but darker than its major counterpart. The lyrics indicate that, like a musical variation, this is an ongoing and recurring thing ("ain't nothing new," "more of the same thing," "it's just a part of the pattern," "it's not as though I don't know the condition").
So he's saying both that it's just a slight (minor) change (variation) in his condition, and that he's in a dark, brooding, minor-keyed mood, and a recurring one at that.