This question already has an answer here:
I have already check this out (Why does the dominant chord contain a flattened 7th?).
I'm not interested in (more or less) dissonant chords, neither in construction of the dominant chords.
My point is: the 7th note (of the C major scale) is B (not B flat) * [C, D, E, F, G, A, B] *
It must be a convention.
That said, does anyone know why (and, if possible, WHEN) this convention was adopted using the "7th flattened" instead of THE REAL 7th note of the scale to form what today we know as C7 (or anything 7)?