The dominant that pulls to E♭, and the first chord of countless blues in B♭.
Index finger barres the A and G strings at the 1st fret
Load B♭7 onto an interactive fretboard, hear every note, and build it into a progression.
Play B♭7 in the Studio →A B♭7 chord contains the notes A#, F, G#, D. It is built from the dominant 7th formula 1 - 3 - 5 - ♭7, with Bb as the root.
E♭ major, most of the time. The ♭7 sits a semitone above E♭'s third and wants to fall into it, which is what gives dominant chords their forward lean. In a twelve-bar blues in B♭ the same chord is the tonic instead, and that tension simply never resolves — which is the point of the form.