do you know anything about the blues theory? (if there is any)

you play notes that dont belong in the scale you are playing over and it stills sound great, for example lets say you play over C major
the notes you have are C,D,E,F,G,A,B
you play a I7-IV7-V7
that would be C7= C-E-G-Bd
F7= F-A-C-Ed
G7= G-B-D-F
you have two notes (Bd,Ed) that dont belong in C major
and if you add the bluenote Gd you have three notes,
and whats the deal with the bluenote? its a diminished 5th
this note is used by metal bands because of its "evil" sound
howcome it sounds so beautiful in blues?