Normally, a blues band includes one guitarist, a drummer, a bass player and a vocalist. Sometimes there is no separate vocalist. Voice is given by one of the people playing the instruments. The rhythm section may need an extra guitar player. The lead guitarst may either play finger-style or with a pick. A glass or metal slide may be used to fret the guitar in order to get a different feeling to the sound. Amplifiers, especially tube amplifiers are important for the guitar to sound better. So, the three instruments that blues must have are:
Guitar is an essential instrument in playing blues. Some of the guitars that are used to play blues are: Fender Stratocaster, Fender Telecaster, Gibson ES 335, Gibson Les Paul, National Steel Resonator Guitar, etc.
Blues cannot be played without drums. Some drummers just use a bass drum, whereas, others use more. There might be some other drumming instruments like cow bell, wash board, and tambourine. These instruments are used by the horn player or the vocalist.
Basses are also a mandatory part of the blues music. You cannot play blues without it. The common models of basses used are Fender Precision Bass and Double String Bass.
If the band is bigger, then you need some additional musicians, like a keyboardist, horn player or violin player. Some bands only use violin to perform solo. The vocalists sometimes have to play harmonica to fill out the sound of blues.