The six-string dominant at the nut, and the IV chord of a B♭ blues.
Index finger barres the top, D and low E strings at the 1st fret
Load F7 onto an interactive fretboard, hear every note, and build it into a progression.
Play F7 in the Studio →A F7 chord contains the notes F, C, D#, A. It is built from the dominant 7th formula 1 - 3 - 5 - ♭7, with F as the root.
It is F major with one finger moved rather than added. The shape frees the fourth string, letting the barre sound E♭ where F major had F, so the note count stays the same. That single change is enough to turn a resting chord into one that points at B♭.