If there’s a single chord here that screams ‘blues’ then this is it! The most resonant dominant chord you can play on the guitar and very easy to play. This is the moveable variant of the open C7 ...

The first 7th chord most of us learn is called just that: C7, G7, A7, etc ... 5th and b7 – but to say 'a flat 7' would be too easy to confuse with the chord A b7, a completely different thing.