I thought C7 meant Cmaj + minor 7, not Cmaj + diminished 7? Or are they the same thing?
actually now that i think of it, diminished 7 actually means double flat 7. or if you are in a reasonable mood, just 6. but for clarity's sake i would prefer to call that Bb a flat 7, instead of saying minor 7, because minor 7 is the name of a chord which also includes the minor 3rd