The 7 Chord is half diminished - a Minor 7 Flat 5, usually written m7b5. But if you're not adding the 7th to your chords, then you're right it would normaly be called diminished.
A Diminished 7th chord would have a double flatted 7th (which is a 6th) and does not occur in the major scale and modes, but it makes a great sub and everyone uses it all over the place!