But if the article of devotion has been blessed by the Sovereign Pontiff or by any Bishop, the faithful, using it, can also gain a plenary indulgence on the feast of the Holy Apostles, Peter and Paul, provided they also make a profession of faith according to any legitimate formula.
The above grant is taken from Norm 17 of the  Apostolic Constitution The Doctrine of Indulgences. Also Norm 19 of the Enchiridion 
Footnote : In order to bless an article or devotion properly the priest uses the prescribed fromula, if there is any; otherwise, he makes a simple sign of the cross toward the article of devotion, laudably adding the words: "In the name of the Father, and of the Son, and of the Holy Spirit". In practice, a formula is to be used in the public blessing of scapulars, while a sign of the cross suffices for other cases.