No, a rosary does not have to be individually blessed to be helpful although the blessing sets aside the object as holy. The prayers that are said using the rosary are what is of primary importance and those prayers are beneficial even if they are said on one’s hands or some other “non-rosary” prayer counter.
As for other sacramentals, it depends on the norms that were put in place when the individual sacramental was established. As one example of a situation in which an individual blessing for the sacramental is not needed, a person who is invested in the brown scapular (also known as the scapular of Our Lady of Mt. Carmel) does not need to have blessed the future brown scapulars that he wears.
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