Bless states that whenever a blessed character makes an action of a certain type, they add 1d4 to that roll. If a character were to use the 1d4 granted to them by this to attack, would that 1d4 be expended indefinitely? Or would that character also add 1d4 to all subsequent attacks until the spell effect ends?
Bless. ...Whenever a target makes an attack roll or a saving throw... the target can roll a d4. (PHB p.219)
The d4 persists until the spell's duration ends or the caster's concentration falls.
Compare with the wording of an effect like Bardic Inspiration (PHB p.53), which spells out that the die is lost upon being used.
The spell's effects continue until the spell ends, either from the caster losing concentration or the duration passing. Nothing in the spell implies that the effect ends after being used any number of times.